vl: don't use 'Yoda conditions'

imitate nearby code about using '!value' or 'value == NULL'
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

vl.c

@@ -1154,7 +1154,7 @@ static int drive_init_func(QemuOpts *opts, void *opaque)
 
 static int drive_enable_snapshot(QemuOpts *opts, void *opaque)
 {
-    if (NULL == qemu_opt_get(opts, "snapshot")) {
+    if (qemu_opt_get(opts, "snapshot") == NULL) {
         qemu_opt_set(opts, "snapshot", "on");
     }
     return 0;
@@ -2506,8 +2506,9 @@ static int foreach_device_config(int type, int (*func)(const char *cmdline))
         loc_push_restore(&conf->loc);
         rc = func(conf->cmdline);
         loc_pop(&conf->loc);
-        if (0 != rc)
+        if (rc) {
             return rc;
+        }
     }
     return 0;
 }