Vm-reg followed by a xmpp-not-ready message causes the client to go away. But,
the pending_vmreg_map_ is not cleaned up when the client is going away. When
the xmpp-not-ready message is later followed by the vm-node create, the code
will attempt to find the client and fail. Fixing this by cleaning up the
pending_vmreg_map_ when we get the xmpp-not-ready message. To do this, we are
keeping track of the each vr-vm reg request in a multimap and we use this to
lookup the vm and cleanup the pending_vmreg_map_.