Get rid of overspecification in vm.xml.

We don't need to pass the appropriate qemu binary into the vm template,
as libvirt will inject the proper one for us.  Similarly, we don't need
to tell libvirt what specific model of PC hardware to emulate, because
it will pick one that is appropriate to the devices we specify.

Change-Id: Id3762cce9b2ba9f96c1e7e79fad9a4bf8770bf7a
Merge "Get rid of overspecification in vm.xml."