ucm: add support for verb variants

ucm: add support for verb variants

The bellow configuration example creates two verbs ("HiFi" and "HiFi 7+1")
with different playback channels for the "Speaker" device.

	SectionUseCase."HiFi" {
	        File "HiFi.conf"
       		Variant."HiFi" {
			Comment "Default"
		}
		Variant."HiFi 7+1" {
			Comment "HiFi 7.1"
        	}
	}

	SectionDevice."Speaker" {
		Value {
			PlaybackChannels 2
		}
		Variant."HiFi 7+1".Value {
			PlaybackChannels 8
		}
	}

Signed-off-by: Jaroslav Kysela <perex@perex.cz>