Make foreach_drive order deterministic

Make foreach_drive order deterministic

Previously, foreach_drive iterated over all configuration
keys (in a random order) and checked whether the current key
is a valid drive name. Instead, we now iterate over a list
of valid drive names (with deterministic order) and check
whether a drive with such a name exists in the
configuration.

Also rename the two involved methods from valid_drive_name
to is_valid_drive_name (for the check) and from disknames
to valid_drive_names (for the list of valid keys), for
consistency. These two were only used in the qemu-server
code base.