python/machine.py: Fix monitor address typing

python/machine.py: Fix monitor address typing

Prior to this, it's difficult for mypy to intuit what the concrete type
of the monitor address is; it has difficulty inferring the type across
two variables.

Create _monitor_address as a property that always returns a valid
address to simplify static type analysis.

To preserve our ability to clean up, use a simple boolean to indicate
whether or not we should try to clean up the sock file after execution.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20201006235817.3280413-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>