virt.qemu_vm: Move get_monitor_filename(s) functions to qemu_monitor

These 2 function are monitor related operation, thus better to put
them in qemu_monitor module.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_monitor: Create 2 functions for creating monitor object

This patch introduces 2 helper functions in qemu_monitor module,
to help user create monitor object.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_vm: Use monitor creation function in qemu_monitor

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_monitor: Store qemu VM object in monitor object

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>