haven: rm: Do not allow concurrent stream from RM

RM will only ever send one stream of fragmented messages to a VM.
Additionally, sequence number should not be used to track notification
fragments. We cannot use IDR sequence pool to associate fragments
with a particular connection. Thus, maintain a current connection to
directly append to when receiving continuation fragments.

Connection streams have a consistent num_fragments and message ID which
can be used to do self-sanity checks instead of sequence number. Use
these fields instead.

Change-Id: I18113868b9fd20d3c9f7f66933cdcfb8c0f29362
Signed-off-by: Elliot Berman <eberman@codeaurora.org>