virt: haven: rm_core: Validate notifications using payload size only

Preparation for cleaning up the resource manager core.
Notification size validation needs to only be performed on the payload
size. The RM RPC header will always be present, else the RM driver would
not have been able to determine that the message is a notification.
Thus, considering the RPC header is not required.

Change-Id: Id6a561a77d693e4f8f307d9d0d96b8d173940c60
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@quicinc.com>