libvirt: Set guest machine type on system z

The domain XML for guests on system z must have the machine type
"s390-ccw-virtio" to set up the device addresses properly. The
default value "s390-virtio" (without "ccw") which is listed by
libvirts get_host_capabilities call cannot be used properly for
guests.

Partial-Implements blueprint libvirt-kvm-systemz

Change-Id: Ib9cfb0ad8db3fe9bfc6b1302761f728fe56c0602
Merge "libvirt: Set guest machine type on system z"