api: create: refactor parameter check logic

api: create: refactor parameter check logic

In preparation to allow passing along certain parameters together with
'archive'. Moving the parameter checks to after the
conflicts-with-'archive' to ensure that the more telling error will
trigger first.

All check helpers should handle empty params fine, but check first
just to make sure and to avoid all the superfluous function calls.

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