qemu: don't fill in nicindexes for session mode libvirtd

Commit 4bbe1029 fixed a problem in commit f7afeddc by moving the call
to virNetDevGetIndex() to a location common to all interface types (so
that the nicindex array would be filled in for macvtap as well as tap
interfaces), but the location was *too* common, as the original call
to virNetDevGetIndex() had been in a section qualified by "if
(cfg->privileged)". The result was that the "fixed" libvirtd would try
to call virNetDevGetIndex() even for session mode libvirtd, and end up
failing with the log message:

  Unable to open control socket: Operation not permitted

To remedy that, this patch qualifies the call to virNetDevGetIndex()
in its new location with cfg->privileged.

This resolves https://bugzilla.redhat.com/show_bug.cgi?id=1198244

(cherry picked from commit 705242f8)