libxl: add support for PVH

libxl: add support for PVH

Since this is something between PV and HVM, it makes sense to put the
setting in place where domain type is specified.
To enable it, use <os><type machine="xenpvh">xenpvh</type></os>. It is
also included in capabilities.xml, for every supported HVM guest type - it
doesn't seems to be any other requirement (besides new enough Xen).

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>