libxl/pvh: force PVH guests to use the xenstore shutdown

libxl/pvh: force PVH guests to use the xenstore shutdown

PVH guests are all required to support the xenstore-based shutdown
signalling, since there is no other way for a PVH guest to be
requested to shut down.

For HVM guests we check whether the guest has installed a PV-on-HVM
interrupt callback; that does not make sense for PVH guests.

So for PVH guests, take the PV path: assume that all PVH guests have
suitable xenstore drivers.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>