scripts/qemustart: more compact rand_mac()

Signed-off-by: Yousong Zhou <[email protected]>

scripts/qemustart

@@ -113,7 +113,7 @@ EOF
 }
 
 rand_mac() {
-	hexdump -n 3 -e '"52:54:00:" 2/1 "%02x:" 1/1 "%02x"' /dev/urandom
+	hexdump -n 3 -e '"52:54:00" 3/1 ":%02x"' /dev/urandom
 }
 
 parse_args() {