qemuDomainBuildNamespace: Make @devPath const

The @devPath variable is not modifiable. It merely just points to
string containing path where private devtmpfs is being
constructed. Make it const so it doesn't look weird that it's not
freed.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Mores <pmores@redhat.com>

src/qemu/qemu_domain.c

@@ -15176,7 +15176,7 @@ qemuDomainBuildNamespace(virQEMUDriverConfigPtr cfg,
                          virDomainObjPtr vm)
 {
     struct qemuDomainCreateDeviceData data;
-    char *devPath = NULL;
+    const char *devPath = NULL;
     char **devMountsPath = NULL, **devMountsSavePath = NULL;
     size_t ndevMountsPath = 0, i;
     int ret = -1;