scripts/device-crash-test: Use a QMP timeout

Despite all the previous fixes, it's still possible for
device-crash-test to wedge itself in the case that QEMU terminates *so
quickly* that it doesn't even begin a connection attempt to our QMP
client. Python will just joyfully wait ad infinitum for a connection
that will now never arrive.

The real fix is to use asyncio to simultaneously poll both the health of
the launched process AND the connection attempt. That's quite a bit more
invasive than just setting a connection timeout, though.

Do the very simplest thing for now.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 20211118204620.1897674-7-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>