[libvirt] libvirt git doesn't know to use -no-kvm on qemu-kvm 1.2

[Cole Robinson]

Libvirt git isn't correctly starting plain qemu guests when using qemu-kvm 1.2
(on Fedora 18). The problem is that -no-kvm is not passed to the binary, so
the default mode is used, which is kvm=enabled.

The issue comes from capabilities probing: we probe against 'qemu-kvm -M
none', which responds to 'query-kvm' with enabled=False, when regular
invocation is enabled=True.

We should be able to work around this by using -machine ...,accel=kvm|tcg
unconditionally for qemu >= 1.2, and not have to do the -enable-kvm/-no-kvm
dance. accel= has been around for a while and libguestfs uses it so it should
be reliable.

- Cole