schema: check format: parse list formats as arrays

schema: check format: parse list formats as arrays

Previously, the returned value would be only the last element or undef in case
of an empty list. There's only a handful of callers of check_format() that look
at the return value and AFAICT none of the exisitng ones is for a -list format.
But best to avoid any future surprises.

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