vsh: Introduce complete command

This command is going to be called from bash completion script in
the following form:

  virsh complete -- start --domain

Its only purpose is to return list of possible strings for
completion. Note that this is a 'hidden', unlisted command and
therefore there's no documentation to it.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

tools/virt-admin.c

@@ -1351,6 +1351,7 @@ static const vshCmdDef vshAdmCmds[] = {
     VSH_CMD_PWD,
     VSH_CMD_QUIT,
     VSH_CMD_SELF_TEST,
+    VSH_CMD_COMPLETE,
     {.name = "uri",
      .handler = cmdURI,
      .opts = NULL,