Add OOM error reporting to a few fucntions

They report errors in all other cases.

src/lxc/lxc_container.c

@@ -223,7 +223,7 @@ static virCommandPtr lxcContainerBuildInitCmd(virDomainDefPtr vmDef,
     }
     virBufferTrim(&buf, NULL, 1);
 
-    if (virBufferError(&buf))
+    if (virBufferCheckError(&buf) < 0)
         return NULL;
 
     virUUIDFormat(vmDef->uuid, uuidstr);

src/qemu/qemu_capabilities.c

@@ -2686,7 +2686,7 @@ virQEMUCapsSaveCache(virQEMUCapsPtr qemuCaps, const char *filename)
     virBufferAdjustIndent(&buf, -2);
     virBufferAddLit(&buf, "</qemuCaps>\n");
 
-    if (virBufferError(&buf))
+    if (virBufferCheckError(&buf) < 0)
         goto cleanup;
 
     xml = virBufferContentAndReset(&buf);