python/qemu/qmp.py: QMP debug with VM label

QEMUMachine writes some messages to the default logger.
But it sometimes hard to read the output if we have requests to
more than one VM.
This patch adds a label to the logger in the debug mode.

Signed-off-by: Oksana Vohchana <ovoshcha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20200316103203.10046-1-ovoshcha@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>