avocado.virt.qemu.machine: Close serial console after power_off()

Close serial console after VM powers off, otherwise Avocado will leave
the process 'nc' running at the end of the execution.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.virt.qemu.machine: Improve logging with QEMU VM PID

Start to keep track of the VM PID and show it in the
VM log header. This way it'll be easier to understand
what happens during a migration (class UUID keeps the
same, but the QEMU instance - PID - changes as we
move from src to dst). Example:

QEMU VM (621efc41 PID: 6391) Migration successful
QEMU VM (621efc41 PID: 6391) Shut down (migration src)
QEMU VM (621efc41 PID: 6408) Login (Remote) -> (hostname=192.168.0.103, username=root, password=123456, port=5007)

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>