virttest: Add virsh qemu-attach and qemu-agent-command wrapper

Add 2 qemu commands wrapper and update qemu_monitor_command to support
extra options.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Will the missing "--hmp" disturb the output of qemu-monitor-commands?
https://github.com/autotest/virt-test/blob/master/virttest/libvirt_vm.py#L1405