Add a generic way for backends to report the default hypervisor
(ie. QEMU) back to the main code.
The direct implemention reflects the current way that the hypervisor
is chosen at configure time (see m4/guestfs-qemu.m4).
For the libvirt backend, we are already getting this from libvirt
domcapabilities, so we can just return that field.
Note this may return NULL (roughly "no data"), particularly in the
libvirt case because that requires us to have launched the appliance
already.
ce4bfa5d08