Error when adding SR-IOV interfaces in SR-IOV context

Error when adding SR-IOV interfaces in SR-IOV context

Method "add_sriov_interfaces" in Libvirt class throws an error when trying
to add SR-IOV interfaces in the XML file.

Function "setup_sriov_context" in SriovContext if passing to
"_enable_interfaces" method the XML file name instead of passing the XML
string.

JIRA: YARDSTICK-1189

Change-Id: If021c102311ecb98cb26934f78067dfc720e4291
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>
(cherry picked from commit 72df0eaac4a2a1f23439af7f249c2b5d18d1b819)