xen/fb: allow xenfb initialization for hvm guests

There is no reasons why an HVM guest shouldn't be allowed to use xenfb.
As a matter of fact ARM guests, HVM from Linux POV, can use xenfb.
Given that no Xen toolstacks configure a xenfb backend for x86 HVM
guests, they are not affected.

Please note that at this time QEMU needs few outstanding fixes to
provide xenfb on ARM:

http://marc.info/?l=qemu-devel&m=138739419700837&w=2



Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
CC: boris.ostrovsky@oracle.com
CC: plagnioj@jcrosoft.com
CC: tomi.valkeinen@ti.com
CC: linux-fbdev@vger.kernel.org
CC: konrad.wilk@oracle.com
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>