viralloc: Report OOM error on failure

Similarly to VIR_STRDUP, we want the OOM error to be reported in
VIR_ALLOC and friends.

tests/xmconfigtest.c

@@ -37,6 +37,8 @@
 #include "viralloc.h"
 #include "virstring.h"
 
+#define VIR_FROM_THIS VIR_FROM_NONE
+
 static virCapsPtr caps;
 static virDomainXMLOptionPtr xmlopt;