linux: Use a separate lock to serialize start/stop vs hotplug events

linux: Use a separate lock to serialize start/stop vs hotplug events

Using one lock for this is a bad idea, as we should not be holding any
locks used by the hotplug thread when trying to stop otherwise the stop
function may wait indefinetely in pthread_join, while the event-thread
is waiting for the lock the caller of the stop function holds.

Using 2 separate locks for this should fix this deadlock, which has been
reported here: https://bugzilla.redhat.com/show_bug.cgi?id=985484

Many thanks to Chris Dickens for figuring out the cause of this deadlock!

CC: Chris Dickens <christopher.a.dickens@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>