virt: gunyah: rm_core: Rename connection variables

Rename "recv_buff" and "recv_buff_size" to "payload" and "size",
respectively. gh_rm_connection on tracks received buffers, so it is
redundant to add "recv". Rename "reply_err_code" to "rm_error" to make
it clear that this field is used for the resource manager's return error
code, and not some other value. While here, re-org and document the
gh_rm_connection struct.

Change-Id: I101262dd907a0e678d8d2c5232a17745f057edaf
Signed-off-by: Elliot Berman <[email protected]>