virThreadPoolFree: Join worker threads

virThreadPoolFree: Join worker threads

Even though currently we are freeing the pool of worker threads at the
daemon very end, nothing holds us back in joining the worker threads.
Moreover, we avoid leaks like this:

==26697== 1,680 bytes in 5 blocks are possibly lost in loss record 913 of 942
==26697==    at 0x4C2BDE4: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==26697==    by 0x4011131: allocate_dtv (in /lib64/ld-2.16.so)
==26697==    by 0x401176D: _dl_allocate_tls (in /lib64/ld-2.16.so)
==26697==    by 0x8499602: pthread_create@@GLIBC_2.2.5 (in /lib64/libpthread-2.16.so)
==26697==    by 0x52F53E9: virThreadCreate (virthreadpthread.c:188)
==26697==    by 0x52F5D4F: virThreadPoolNew (virthreadpool.c:221)
==26697==    by 0x53F30DB: virNetServerNew (virnetserver.c:377)
==26697==    by 0x11C6ED: main (libvirtd.c:1366)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>