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>
libvirt_vm: Add '--hmp' option for get_vcpus_pid function

As updated the qemu_monitor_command, the get_vcpus_pid function is
affected as it use hmp command, so add the option '--hmp' for run
the command.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #1666 from waynesun09/qemu_command

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