xl: create VFB for PV guest when VNC is specified

xl: create VFB for PV guest when VNC is specified

This replicates a Xend behavior. When you specify 'vnc=1' and there's no
'vfb=[]' in a PV guest's config file, xl parses all top level VNC options and
creates a VFB for you.

Fixes bug #25.
http://bugs.xenproject.org/xen/bug/25

Reported-by: Konrad Wilk <konrad.wilk@oracle.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>