storage: Don't assume storage pool exists for FC/SCSI refresh thread

storage: Don't assume storage pool exists for FC/SCSI refresh thread

https://bugzilla.redhat.com/show_bug.cgi?id=1277781

The virStoragePoolFCRefreshThread had passed a pointer to the pool obj
in the virStoragePoolFCRefreshInfoPtr; however, we cannot assume that
the pool exists still since we don't keep the pool lock throughout
the duration of the thread.

Therefore, instead of passing the pool obj pointer, pass the UUID of
the pool and perform a lookup.  If found, then we can perform the
refresh using the locked pool obj pointer; otherwise, we just exit
the thread since the pool is now gone.