core: fix NPEx when adding new vm pool

This patch adds handling of null value of the soundcard attribute when
creating a new VM pool. Without it the creation of the VM pool was
throwing NPEx when the attribute wasn't specified (tried with restapi).

Change-Id: I51262b8ceec47ab4e36b0180ec8d3fd9af6033b3
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>