pvh: introduce PVH guest type

pvh: introduce PVH guest type

Introduce new PVH guest type, flags to create it, and ways to identify it.

To begin with, it will inherit functionality marked hvm_container.

Code to actually check for hardware support, in the VMX case, will be added
in future patches.

Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Eddie Dong <eddie.dong@intel.com>