qemu: Actually unshare() iff running as root

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>

src/qemu/qemu_domain.c

@@ -7400,6 +7400,9 @@ qemuDomainBuildNamespace(virQEMUDriverPtr driver,
         goto cleanup;
     }
 
+    if (virProcessSetupPrivateMountNS() < 0)
+        goto cleanup;
+
     if (qemuDomainSetupDev(driver, vm, devPath) < 0)
         goto cleanup;
 

src/qemu/qemu_process.c

@@ -2663,9 +2663,6 @@ static int qemuProcessHook(void *data)
     if (virSecurityManagerClearSocketLabel(h->driver->securityManager, h->vm->def) < 0)
         goto cleanup;
 
-    if (virProcessSetupPrivateMountNS() < 0)
-        goto cleanup;
-
     if (qemuDomainBuildNamespace(h->driver, h->vm) < 0)
         goto cleanup;