xenfb2: Amend default framebuffer values.
xenfb2: Amend default framebuffer values.

1280x1024 32bpp default stride should be 5120 minimum, 4096 is just not
enough to represent a pixel line. This leads to plugins reporting the
initial framebuffer as invalid:
> [PLUGIN-DRM] drmModeAddFB(/dev/dri/card0, 1280x1024:24/32 pitch:4096) failed (Invalid argument).
> [drm:drm_internal_framebuffer_create [drm]] bad pitch 4096 for plane 0

Since Xenfb2 is being loaded fairly late there is no console to display
anyway, but the error should not trigger anyway.

Signed-off-by: Eric Chanudet <chanudete@ainfosec.com>

OXT-1639

OpenXT/surfman,OpenXT/surfman,OpenXT/surfman