Introduce a new flag for i440fx to disable PCI hotplug on the root bus

We introduce a new global flag 'acpi-root-pci-hotplug' for i440fx with which
we can turn on or off PCI device hotplug on the root bus. This flag can be
used to prevent all PCI devices from getting hotplugged or unplugged from the
root PCI bus.
This feature is targetted mostly towards Windows VMs. It is useful in cases
where some hypervisor admins want to deploy guest VMs in a way so that the
users of the guest OSes are not able to hot-eject certain PCI devices from
the Windows system tray. Laine has explained the use case here in detail:
https://www.redhat.com/archives/libvir-list/2020-February/msg00110.html

Julia has resolved this issue for PCIE buses with the following commit:
530a0963184e57e71a5b538 ("pcie_root_port: Add hotplug disabling option")

This commit attempts to introduce similar behavior for PCI root buses used in
i440fx machine types (although in this case, we do not have a per-slot
capability to turn hotplug on or off).

Usage:
   -global PIIX4_PM.acpi-root-pci-hotplug=off

By default, this option is enabled which means that hotplug is turned on for
the PCI root bus.

The previously existing flag 'acpi-pci-hotplug-with-bridge-support' for PCI-PCI
bridges remain as is and can be used along with this new flag to control PCI
hotplug on PCI bridges.

This change has been tested using a Windows 2012R2 server guest image and also
with a Windows 2019 server guest image on a Ubuntu 18.04 host using the latest
master qemu from upstream.

Signed-off-by: Ani Sinha <ani@anisinha.ca>
Message-Id: <20200821165403.26589-1-ani@anisinha.ca>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>

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