virt: haven: rm_core: Always allocate a connection for notifications

Currently, there is an optimized path to directly use the message queue
buffer when a received notification fits within a single message and
there aren't any fragments. This greatly complicates buffer management
and might not provide any measurable benefit.
Suppose Linux receives a notification with one fragment then a
notification with 3 fragments. In current flow, there is 1 alloc for the
first notification and 4 allocations for the 2nd notification (1 for
each message + 1 for overall combined buffer). In the new flow, we could
optimize this to 0 allocations for each message because we can re-use
the same message buffer for each fragment and then a single alloc for
the entire payload.
There is, however, a slightly increased cost for single-message
notifications in the form of an extra memcpy from the message buffer to
notification buffer. Under the code optimizations and simplifications
that come with this cost, it is a worthy cost.

Change-Id: Ib52bd332f59b108d7dab9121c0beefed0aa66994
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@quicinc.com>