vnic QoS: Fix specParams to XML definition conversion

VDSM API specifies specParams input and output data to have values
specified as integers. In order to generate valid XML, those values
should have been converted to strings (and the test should have been
passing integers as per VDSM schema).

This patch fixes the conversion code and the tests for it (fix one,
add a new one for the update flow).

Change-Id: I25041b8f853c870388ecd9c9f9176bd384242234
Bug-Url: https://bugzilla.redhat.com/1067064
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25788
Reviewed-by: Dan Kenigsberg <danken@redhat.com>