amidi: Ignore inactive MIDI ports as default at listing

amidi: Ignore inactive MIDI ports as default at listing

When listing the devices, currently we show all MIDI ports including
inactive ones.  But those inactive ports are rarely useful, and it'd
be more convenient to filter them out.

This patch introduces the filtering of inactive ports at listing
devices via amidi -l option.  When user needs to scan all MIDI ports
including inactive ports, pass the new option -x in addition.

Signed-off-by: Takashi Iwai <tiwai@suse.de>