libvhost-user: Add vu_rem_mem_reg input validation

libvhost-user: Add vu_rem_mem_reg input validation

Today if multiple FDs are sent from the VMM to the backend in a
VHOST_USER_REM_MEM_REG message, one FD will be unmapped and the remaining
FDs will be leaked. Therefore if multiple FDs are sent we report an
error and fail the operation, closing all FDs in the message.

Likewise in case the VMM sends a message with a size less than that of a
memory region descriptor, we add a check to gracefully report an error
and fail the operation rather than crashing.

Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20220117041050.19718-2-raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>