python: backport 'avoid creating additional event loops per thread'

python: backport 'avoid creating additional event loops per thread'

This commit is two backports squashed into one to avoid regressions.

python: *really* remove get_event_loop

A prior commit, aa1ff990, switched away from using get_event_loop *by
default*, but this is not good enough to avoid deprecation warnings as
`asyncio.get_event_loop_policy().get_event_loop()` is *also*
deprecated. Replace this mechanism with explicit calls to
asyncio.get_new_loop() and revise the cleanup mechanisms in __del__ to
match.

python: avoid creating additional event loops per thread

"Too hasty by far!", commit 21ce2ee4 attempted to avoid deprecated
behavior altogether by calling new_event_loop() directly if there was no
loop currently running, but this has the unfortunate side effect of
potentially creating multiple event loops per thread if tests
instantiate multiple QMP connections in a single thread. This behavior
is apparently not well-defined and causes problems in some, but not all,
combinations of Python interpreter version and platform environment.

Partially revert to Daniel Berrange's original patch, which calls
get_event_loop and simply suppresses the deprecation warning in
Python<=3.13. This time, however, additionally register new loops
created with new_event_loop() so that future calls to get_event_loop()
will return the loop already created.

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
cherry picked from commit python-qemu-qmp@21ce2ee4f2df87efe84a27b9c5112487f4670622
cherry picked from commit python-qemu-qmp@c08fb82b38212956ccffc03fc6d015c3979f42fe
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>