vmspawn: pass --log-level=error and --modcaps=-mknod to virtiofsd

Reduce virtiofsd log noise by setting --log-level=error, and drop
the unnecessary mknod capability with --modcaps=-mknod, matching
mkosi's virtiofsd invocation.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>

src/vmspawn/vmspawn.c

@@ -1700,7 +1700,9 @@ static int start_virtiofsd(
                         "--shared-dir", source_uid == FOREIGN_UID_MIN ? "/run/systemd/mount-rootfs" : directory,
                         "--xattr",
                         "--fd", sockstr,
-                        "--no-announce-submounts");
+                        "--no-announce-submounts",
+                        "--log-level=error",
+                        "--modcaps=-mknod");
         if (!argv)
                 return log_oom();