https://bugzilla.redhat.com/show_bug.cgi?id=1413922

While all the code that deals with qemu namespaces correctly
detects whether we are running as root (and turn into NO-OP for
qemu:///session) the actual unshare() call is not guarded with
such check. Therefore any attempt to start a domain under
qemu:///session shall fail as unshare() is reserved for root.

The fix consists of moving unshare() call (for which we have a
wrapper called virProcessSetupPrivateMountNS) into
qemuDomainBuildNamespace() where the proper check is performed.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Tested-by: NRichard W.M. Jones <rjones@redhat.com>