util: Introduce and use virObjectRWUnlock

util: Introduce and use virObjectRWUnlock

Rather than overload virObjectUnlock as commit id '77f4593b' has
done, create a separate virObjectRWUnlock API that will force the
consumers to make the proper decision regarding unlocking the
RWLock's. Similar to the RWLockRead and RWLockWrite, use the
virObjectGetRWLockableObj helper. This restores the virObjectUnlock
code to using the virObjectGetLockableObj.

Signed-off-by: John Ferlan <jferlan@redhat.com>