Add virsh qemu-agent-command test cases

The virsh qemu-agent-command is for send QEMU agent command directly
from virsh, which benefit debugging for devel or well knowledged user.

The qemu-* commands are not with support in RHEL products, so the testing
strategy is mainly focus on return status and libvirtd status after
command been executed.

Signed-off-by: Wayne Sun <gsun@redhat.com>