docs/microvm.rst: document the new microvm machine type

Document the new microvm machine type.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

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+====================
+microvm Machine Type
+====================
+
+``microvm`` is a machine type inspired by ``Firecracker`` and
+constructed after its machine model.
+
+It's a minimalist machine type without ``PCI`` nor ``ACPI`` support,
+designed for short-lived guests. microvm also establishes a baseline
+for benchmarking and optimizing both QEMU and guest operating systems,
+since it is optimized for both boot time and footprint.
+
+
+Supported devices
+-----------------
+
+The microvm machine type supports the following devices:
+
+- ISA bus
+- i8259 PIC (optional)
+- i8254 PIT (optional)
+- MC146818 RTC (optional)
+- One ISA serial port (optional)
+- LAPIC
+- IOAPIC (with kernel-irqchip=split by default)
+- kvmclock (if using KVM)
+- fw_cfg
+- Up to eight virtio-mmio devices (configured by the user)
+
+
+Limitations
+-----------
+
+Currently, microvm does *not* support the following features:
+
+ - PCI-only devices.
+ - Hotplug of any kind.
+ - Live migration across QEMU versions.
+
+
+Using the microvm machine type
+------------------------------
+
+Machine-specific options
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+It supports the following machine-specific options:
+
+- microvm.x-option-roms=bool (Set off to disable loading option ROMs)
+- microvm.pit=OnOffAuto (Enable i8254 PIT)
+- microvm.isa-serial=bool (Set off to disable the instantiation an ISA serial port)
+- microvm.pic=OnOffAuto (Enable i8259 PIC)
+- microvm.rtc=OnOffAuto (Enable MC146818 RTC)
+- microvm.auto-kernel-cmdline=bool (Set off to disable adding virtio-mmio devices to the kernel cmdline)
+
+
+Boot options
+~~~~~~~~~~~~
+
+By default, microvm uses ``qboot`` as its BIOS, to obtain better boot
+times, but it's also compatible with ``SeaBIOS``.
+
+As no current FW is able to boot from a block device using
+``virtio-mmio`` as its transport, a microvm-based VM needs to be run
+using a host-side kernel and, optionally, an initrd image.
+
+
+Running a microvm-based VM
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+By default, microvm aims for maximum compatibility, enabling both
+legacy and non-legacy devices. In this example, a VM is created
+without passing any additional machine-specific option, using the
+legacy ``ISA serial`` device as console::
+
+  $ qemu-system-x86_64 -M microvm \
+     -enable-kvm -cpu host -m 512m -smp 2 \
+     -kernel vmlinux -append "earlyprintk=ttyS0 console=ttyS0 root=/dev/vda" \
+     -nodefaults -no-user-config -nographic \
+     -serial stdio \
+     -drive id=test,file=test.img,format=raw,if=none \
+     -device virtio-blk-device,drive=test \
+     -netdev tap,id=tap0,script=no,downscript=no \
+     -device virtio-net-device,netdev=tap0
+
+While the example above works, you might be interested in reducing the
+footprint further by disabling some legacy devices. If you're using
+``KVM``, you can disable the ``RTC``, making the Guest rely on
+``kvmclock`` exclusively. Additionally, if your host's CPUs have the
+``TSC_DEADLINE`` feature, you can also disable both the i8259 PIC and
+the i8254 PIT (make sure you're also emulating a CPU with such feature
+in the guest).
+
+This is an example of a VM with all optional legacy features
+disabled::
+
+  $ qemu-system-x86_64 \
+     -M microvm,x-option-roms=off,pit=off,pic=off,isa-serial=off,rtc=off \
+     -enable-kvm -cpu host -m 512m -smp 2 \
+     -kernel vmlinux -append "console=hvc0 root=/dev/vda" \
+     -nodefaults -no-user-config -nographic \
+     -chardev stdio,id=virtiocon0 \
+     -device virtio-serial-device \
+     -device virtconsole,chardev=virtiocon0 \
+     -drive id=test,file=test.img,format=raw,if=none \
+     -device virtio-blk-device,drive=test \
+     -netdev tap,id=tap0,script=no,downscript=no \
+     -device virtio-net-device,netdev=tap0