vfio/igd: Do not include GTT stolen size in etc/igd-bdsm-size

vfio/igd: Do not include GTT stolen size in etc/igd-bdsm-size

Though GTT Stolen Memory (GSM) is right below Data Stolen Memory (DSM)
in host address space, direct access to GSM is prohibited, and it is
not mapped to guest address space. Both host and guest accesses GSM
indirectly through the second half of MMIO BAR0 (GTTMMADR).

Guest firmware only need to reserve a memory region for DSM and program
the BDSM register with the base address of that region, that's actually
what both SeaBIOS[1] and IgdAssignmentDxe does now.

[1] https://gitlab.com/qemu-project/seabios/-/blob/1.12-stable/src/fw/pciinit.c#L319-332

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
Link: https://lore.kernel.org/qemu-devel/20250306180131.32970-3-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>