vfio/igd: don't set stolen memory size to zero

vfio/igd: don't set stolen memory size to zero

The stolen memory is required for the GOP (EFI) driver and the Windows
driver. While the GOP driver seems to work with any stolen memory size,
the Windows driver will crash if the size doesn't match the size
allocated by the host BIOS. For that reason, it doesn't make sense to
overwrite the stolen memory size. It's true that this wastes some VM
memory. In the worst case, the stolen memory can take up more than a GB.
However, that's uncommon. Additionally, it's likely that a bunch of RAM
is assigned to VMs making use of GPU passthrough.

Signed-off-by: Corvin Köhne <c.koehne@beckhoff.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>