config list helper: fix return type of vmid for container index API enpoint

config list helper: fix return type of vmid for container index API enpoint

The schema declares it to be an integer. Done in the config_list()
helper, to ensure consistency for the following existing code in
vmstatus():

> my $list = $opt_vmid ? { $opt_vmid => { type => 'lxc', vmid => int($opt_vmid) }} : config_list();

Should not cause any issues for other callers of the helper.

Reported in the community forum:
https://forum.proxmox.com/threads/137628/

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>