virtio_gpu: use an X8R8G8B8 resource on big-endian guests

The virtio-gpu resource formats are defined by byte order in memory,
while vt(4) and the X server (through vt_fb and the fb mmap) write
nati

virtio_gpu: use an X8R8G8B8 resource on big-endian guests

The virtio-gpu resource formats are defined by byte order in memory,
while vt(4) and the X server (through vt_fb and the fb mmap) write
native 32-bit 0x00RRGGBB pixels into the shadow framebuffer. On a
big-endian guest such as powerpc64 those pixels land in memory as
00 RR GG BB, which the host interprets under B8G8R8X8 with red and
blue swapped and the padding byte taken as blue: white renders yellow
and blue renders black.

Request X8R8G8B8 on big-endian guests instead, which is also what the
Linux driver does (DRM_FORMAT_HOST_XRGB8888). Reproduced with a colour
test pattern on a powerpc64 QEMU pseries guest using the identically
coded out-of-tree fork of this driver (graphics/virtio-gpu-qemu-kmod);
little-endian guests are unchanged.

Differential Revision: https://reviews.freebsd.org/D60085
Approved by: jhibbits
MFC after: 1 week

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