Add qemu-* command test cases

The qemu-* commands are explicitly unsupported in RHEL products, so
the testing strategy is mainly focus on return status and libvirtd status after
command been executed.

This depends on PR:
https://github.com/autotest/virt-test/pull/1666
@waynesun09 , sorry, I introduced a wrong module, it should be qemu_vm.py.
My intention is to use exist module to create the VM running by "qemu-kvm xxx".
And to get the pid is much easier with function get_pid().