Fix for #5836 (--lock-many with vms)

This makes --lock-many work when --machine-type vps is passed.
Before it wasn't handled correctly and guests were not created.
Now it creates and gives the back the user the list-targets for
said guests.

teuthology-lock --lock-many 4 --machine-type vps --os-type centos

This fixes issue #5836

Signed-off-by: Sandon Van Ness <sandon@inktank.com>
Reviewed-by: Alfredo Deza <alfredo@deza.pe>