gdb/python: generalize serialize_mi_result()

gdb/python: generalize serialize_mi_result()

This commit generalizes serialize_mi_result() to make usable in
different contexts than printing result of custom MI command.

To do so, the check whether passed Python object is a dictionary has been
moved to the caller - at the very least, different uses require different
error messages.  Also it has been renamed to serialize_mi_results() to better
match GDB/MI output syntax (see corresponding section in documentation,
in particular rules 'result-record' and 'async-output'.

Since it is now more generic function, it has been moved to py-mi.c.

This is a preparation for implementing Python support for sending custom
MI async events.

Approved-By: Andrew Burgess <aburgess@redhat.com>