python: backport 'Use @asynciocontextmanager'

python: backport 'Use @asynciocontextmanager'

This removes a non-idiomatic use of a "coroutine callback" in favor of
something a bit more standardized.

Signed-off-by: John Snow <jsnow@redhat.com>
cherry picked from commit python-qemu-qmp@commit 97f7ffa3be17a50544b52767d14b6fd478c07b9e
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
(cherry picked from commit 0408b8d7a086486f5c1887798be744b2d73bcda9)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>