Add missing check for OOM when building boot menu args

When building boot menu args, if OOM occurred the CLI args
would end up containing  'order=(null)' due to a missing
call to 'virBufferError'.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/qemu/qemu_command.c

@@ -8057,6 +8057,11 @@ qemuBuildCommandLine(virConnectPtr conn,
         if (boot_nparams > 0) {
             virCommandAddArg(cmd, "-boot");
 
+            if (virBufferError(&boot_buf)) {
+                virReportOOMError();
+                goto error;
+            }
+
             if (boot_nparams < 2 || emitBootindex) {
                 virCommandAddArgBuffer(cmd, &boot_buf);
                 virBufferFreeAndReset(&boot_buf);