virt: migration: use contextmanager for monitor

This patch factors the monitor thread
control in a context manager to make code
clearer and less cluttered.

Change-Id: Ie7fcd0bedf4f30cc0bcab03339322df1fc5434e8
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25978
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>