virsh: undefine: Clarify help string for --snapshots-metadata

Reword the end of the help string to make it more obvious that the VM
must be inactive.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NFabiano Fidêncio <fidencio@redhat.com>
ACKed-by: NEric Blake <eblake@redhat.com>

tools/virsh-domain.c

@@ -3627,7 +3627,7 @@ static const vshCmdOptDef opts_undefine[] = {
     },
     {.name = "snapshots-metadata",
      .type = VSH_OT_BOOL,
-     .help = N_("remove all domain snapshot metadata, if inactive")
+     .help = N_("remove all domain snapshot metadata (vm must be inactive)")
     },
     {.name = "nvram",
      .type = VSH_OT_BOOL,