xenapi: xe network-attach for OVS bridge creation

For Neutron we need an additional OVS bridge to be present. The xe
network-create command did not create the bridge immediately. A
workaround was applied to attach a network interface to that network,
forcing xapi to create the underlying OVS bridge. It turned out, that
the xe network-attach command could be used instead, so the workaround
is removed and replaced with the proper solution by this patch.

Fixes bug 1197305

Change-Id: I10cc763531b71238f0bc12a12de985f8f35d7e27
Merge "xenapi: xe network-attach for OVS bridge creation"