swtpm_setup: Add missing description for --vmid to help screen

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
diff --git a/src/swtpm_setup/swtpm_setup.c b/src/swtpm_setup/swtpm_setup.c
index 81f096e..1b528c8 100644
--- a/src/swtpm_setup/swtpm_setup.c
+++ b/src/swtpm_setup/swtpm_setup.c
@@ -887,6 +887,8 @@
         "\n"
         "--not-overwrite  : Do not overwrite existing TPM state but silently end\n"
         "\n"
+        "--vmid <vm id>   : Unique (VM) identifier to use as common name in certificate\n"
+        "\n"
         "--pcr-banks <banks>\n"
         "                 : Set of PCR banks to activate. Provide a comma separated list\n"
         "                   like 'sha1,sha256'. '-' to skip and leave all banks active.\n"