virt: gunyah: rm_core: Guard against unexpected messages

Gracefully handle non-standard sequences of messages such as
continuation messages without receiving a reply/notification message
first, or a new reply/notification message without having finished the
last message sequence.

Change-Id: I52922fc9ccefb1f7dbb40fbffd2ad6b7fd1162b7
Signed-off-by: Elliot Berman <[email protected]>