logging: consistently retrieve source ID from remote messages

The log message header "source" field for messages received from a
remote domain contains the source ID, rather than a pointer to the
source data (which would not be valid in the local domain).

msg_filter_check() did not handle this case and obtained a garbage source
ID for remote log messages. This caused an assertion failure in
filter_get().

Consistently handle this by adding a log_msg_get_source_id() function
that returns the source ID for both local and remote messages. This
function was implemented based on code factored out of
log_output_msg_process().

Signed-off-by: Ben Wolsieffer <benwolsieffer@gmail.com>