virt.qemu_monitor: Store qemu VM's name in monitor object

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_monitor: Add a vm_is_dead_func member for Monitor class

This member normally points to the vm.is_dead method, so that we
can check whether vm is alive in monitor class.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_monitor: Check if monitor is dead before receiving msg from it

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