s390x/kvm: make use of generic vm attribute check

By using the new introduced generic interface we
can remove redundancies and clean up.

Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Suggested-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Message-Id: <1426164834-38648-3-git-send-email-jfrei@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
