tests/migration: fix unix socket batch migration

tests/migration: fix unix socket batch migration

when execute the following test command:
"guestperf-batch.py --dst-host localhost --transport unix ..."
test aborts and error message as the following be throwed:
"launching VM Failed: [Errno 98] Address already in use".

The reason is that batch script use the same monitor socket
in all test cases and do not remove the socket file. The second
migration test will launch vm use the same socket file as
the first, so we get the error message. To fix it, just remove
the socket file each time we have done the migration test.

Signed-off-by: Hyman <huangy81@chinatelecom.cn>
Message-Id: <c3fc438993b87a6ab0bea3d07f6ca0260d29936e.1615397103.git.huangy81@chinatelecom.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>