hw/xen/xen_pt_graphics: Don't trust the BIOS ROM contents so much

Coverity (CID 796599) points out that xen_pt_setup_vga() trusts
the rom->size field in the BIOS ROM from a PCI passthrough VGA
device, and uses it as an index into the memory which contains
the BIOS image. A corrupt BIOS ROM could therefore cause us to
index off the end of the buffer.

Check that the size is within bounds before we use it.

We are also trusting the pcioffset field, and assuming that
the whole rom_header is present; Coverity doesn't notice these,
but check them too.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

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