Add a null undo function to the vmbackup null provider.

Add a null undo function to the vmbackup null provider.

If a snapshot operation times out, vmbackup can attempt
to undo quiescing.  Since no quiescing is done for the null
backup provider, no undo function was provided.  If vmbackup
attempts to call the undo function, it dereferences a garbage
pointer resulting in a segfault.

Rather than add null backup provider specific checks to vmbackup,
this change adds a null undo function to provide vmbackup with a
valid function pointer it can call.  The new undo function updates
the vmbackup state machine state with a new currentOpName, but
has no other effect.  currentOpName is set to the calling
function name, e.g. __FUNCTION__.