api: create_vm: check serial and usb permissions

api: create_vm: check serial and usb permissions

The existing check_vm_modify_config_perm doesn't do so anymore, but
the check only got re-added to the modify/delete paths. See commits
165be267ebff3f614cc6b56bbb594ef4d456a4e9 and
e30f75c571a6a678f8f1f3dfb2dee1b622b49185 for context.

In the future, it might make sense to generalise the
check_vm_modify_config_perm and have it not only take keys, but both
new and old values, and use that generalised function everywhere.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>