virt: gunyah: rm_core: Use a single buffer to process messages from RM

The message buffer received by gh_msgq_recv is now not directly used by
any upper layers and always copied to a "buff" in gh_rm_connection, so
the same buffer can be re-used in the gh_msgq_recv loop.

Change-Id: I92988711b84b6f1e6b653c7d8cb5b66abc7a322e
Signed-off-by: Elliot Berman <[email protected]>