vfio/igd: add x-igd-gms option back to set DSM region size for guest

vfio/igd: add x-igd-gms option back to set DSM region size for guest

DSM region is likely to store framebuffer in Windows, a small DSM
region may cause display issues (e.g. half of the screen is black).
Since 971ca22f041b ("vfio/igd: don't set stolen memory size to zero"),
the x-igd-gms option was functionally removed, QEMU uses host's
original value, which is determined by DVMT Pre-Allocated option in
Intel FSP of host bios.

However, some vendors do not expose this config item to users. In
such cases, x-igd-gms option can be used to manually set the data
stolen memory size for guest. So this commit brings this option back,
keeping its old behavior. When it is not specified, QEMU uses host's
value.

When DVMT Pre-Allocated option is available in host BIOS, user should
set DSM region size there instead of using x-igd-gms option.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20241206122749.9893-11-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>