Revert "xen/fb: allow xenfb initialization for hvm guests"

Revert "xen/fb: allow xenfb initialization for hvm guests"

This reverts commit d9581c7dcac15c02ad4d47c60c60f4d8f197db55.

which is:
Author: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Date:   Fri Jan 3 19:02:09 2014 +0000

    xen/fb: allow xenfb initialization for hvm guests

    There is no reasons why an HVM guest shouldn't be allowed to use xenfb.

as "Xend" exposes an PV and VGA (via QEMU) driver to the guests (either
PV or HVM).

The backed in this case is QEMU - and it only provides the emulation
for the VGA backend (for HVM guests). Upstream QEMU can do both - but
since we are using Xend - we only expose QEMU-traditional.

Upstreamwise we hadn't reached a good decisions. Poking for XenStore
keys to see if the backend is Xend vs xl seems odd. And the issue
goes away in 'xl' as you can't define both PV and VGA drivers. Only
one of them is allowed.

The 'proper' fix would be to teach Xend to not expose PV FB for HVM
guests but that code is pretty genric.

So continuing on with this revert.

OraBug: 20386370 - XENBUS_PROBE_FRONTEND: TIMEOUT CONNECTING TO DEVICE EROR WITH UEK4
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>