Apparently, VBE maps the VGA vram to a fixed physical location.  KVM requires
that all mappings of the VGA vram have dirty tracking enabled on them.  Any
access to the VGA vram through the VBE mapping currently fails to result in
dirty page tracking updates causing a black screen.

This is the true root cause of VMware VGA not working correctly under KVM and
likely also an issue with some of the std-vga black screen issues too.

Cirrus does not enable VBE so it would not be a problem when using Cirrus.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Rebased-by: Dave Airlie <airlied@redhat.com>