When you've got Qsynth running, do "aconnect -oil" to see a list of ALSA sequencer clients. For example
$ aconnect -oil
client 0: 'System' [type=kernel]
0 'Timer '
1 'Announce '
client 14: 'Midi Through' [type=kernel]
0 'Midi Through Port-0'
client 16: 'Virtual Raw MIDI 0-0' [type=kernel]
0 'VirMIDI 0-0 '
client 17: 'Virtual Raw MIDI 0-1' [type=kernel]
0 'VirMIDI 0-1 '
client 18: 'Virtual Raw MIDI 0-2' [type=kernel]
0 'VirMIDI 0-2 '
client 19: 'Virtual Raw MIDI 0-3' [type=kernel]
0 'VirMIDI 0-3 '
client 28: 'FastTrack Pro' [type=kernel]
0 'FastTrack Pro MIDI 1'
client 128: 'FLUID Synth (Qsynth1)' [type=user]
0 'Synth input port (Qsynth1:0)'
1 'Synth input port (Qsynth1:1)'