domain_descriptor: More careful falsy check

Using "if self._devices" works with xml.minidom, but if we switch our xml
library to xml.etree, empty element is considered falsy, which will
create same hash for different xml.

While this may be a non-issue, using "if self._devices is not None" is
more correct and predictable.

Change-Id: Id805237342b3e00334e0e540ce8f4e919cf0107e
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34623
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>