core: fix edit of sound card for running vm

Currently, when updating the sound card setting for running vm,
the query that checks if sound card exists is wrong, because sound card
can have multiple types of devices (ac97,ich6)

Added a code that use the "UNKNOWN" device type,
to mark the mechanism to use only the general device type (sound)
without the specific type.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1128416
Change-Id: If2ff54c3d0929ddf04f39bb42e61e185005d9655
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>