xen/pass-through: don't create needless register group

xen/pass-through: don't create needless register group

Currently we are creating a register group for the Intel IGD OpRegion
for every device we pass through, but the XEN_PCI_INTEL_OPREGION
register group is only valid for an Intel IGD. Add a check to make
sure the device is an Intel IGD and a check that the administrator has
enabled gfx_passthru in the xl domain configuration. Require both checks
to be true before creating the register group. Use the existing
is_igd_vga_passthrough() function to check for a graphics device from
any vendor and that the administrator enabled gfx_passthru in the xl
domain configuration, but further require that the vendor be Intel,
because only Intel IGD devices have an Intel OpRegion. These are the
same checks hvmloader and libxl do to determine if the Intel OpRegion
needs to be mapped into the guest's memory. Also, move the comment
about trapping 0xfc for the Intel OpRegion where it belongs after
applying this patch.

Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <c76dff6369ccf2256bd9eed5141da1db767293d2.1656480662.git.brchuckz@aol.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>