python: only check qemu/ subdir with flake8

flake8 is a little eager to check everything it can. Limit it to
checking inside the qemu namespace directory only. Update setup.cfg now
that the exclude patterns are no longer necessary.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Tested-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-id: 20210629214323.1329806-11-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>