nodedev: Don't join not spawned threads

nodedev: Don't join not spawned threads

During the nodedev driver initialization two threads are created:
one for listening on udev events (like device plug/unplug) and
the other for enumerating devices (so that the main thread doing
the driver init is not blocked). If something goes wrong at any
point then nodeStateCleanup() is called which joins those two
threads (possibly) created before. But it tries to join them even
they weren't created which is undefined behaviour (and it just so
happens that it crashes on my system).

If those two virThread variables are turned into pointers then we
can use comparison against NULL to detect whether threads were
created.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>