modules/virt.py: use qemu-img to detect image type; allow emulator type ...

On Ubuntu 12.04, the output of "file image.qcow2" is not properly parsed by virt.py. This commit fixes that by using qemu-img instead.

In addition to kvm, libvirt also supports qemu as an emulator. This commit allows the emulator type to be passed as arguments to the module (still defaulting to kvm). Note that runners/virt.py has not been updated, as salt-run seems to have an issue with named arguments. I will open a separate issue on that.