python: backport 'qmp-tui: Do not crash if optional dependencies are not met'

python: backport 'qmp-tui: Do not crash if optional dependencies are not met'

Based on the discussion at https://github.com/pypa/pip/issues/9726 -
even though the setuptools documentation implies that it is possible to
guard script execution with optional dependency groups, this is not true
in practice with the scripts generated by pip.

Just do the simple thing and guard the import statements.

Signed-off-by: John Snow <jsnow@redhat.com>
cherry picked from commit python-qemu-qmp@df520dcacf9a75dd4c82ab1129768de4128b554c
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>