selinux: properly label tap FDs with imagelabel

The cleanup in commit cf976d9d used secdef->label to label the tap
FDs, but that is not possible since it's process-only label (svirt_t)
and not a object label (e.g. svirt_image_t).  Starting a domain failed
with EPERM, but simply using secdef->imagelabel instead of
secdef->label fixes it.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/security/security_selinux.c

@@ -2340,7 +2340,7 @@ virSecuritySELinuxSetTapFDLabel(virSecurityManagerPtr mgr ATTRIBUTE_UNUSED,
     if (!secdef || !secdef->label)
         return 0;
 
-    return virSecuritySELinuxFSetFilecon(fd, secdef->label);
+    return virSecuritySELinuxFSetFilecon(fd, secdef->imagelabel);
 }
 
 static char *