config: parse: also allow empty values

config: parse: also allow empty values

because they are valid for '-list' formats and it makes the behavior match with
what we do for VM configs. The new pattern is the same that is used for VM
configs. Because it is a non-greedy pattern, trailing whitespaces will not be
included in the value anymore. This /should/ cause no problems and the '\s*$'
at the end suggests that that is how it was intended in the first place.

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