kvm: encapsulate HAS_DEVICE for vm attrs

More and more virtual machine specifics between kvm and qemu will be
transferred with vm attributes.
So we encapsulate the common logic in a generic function.

Additionally we need only to check during initialization if kvm supports
virtual machine attributes.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-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: <1426164834-38648-2-git-send-email-jfrei@linux.vnet.ibm.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
