datastore: factor type out of ListGroups into ListGroupsType

datastore: factor type out of ListGroups into ListGroupsType

In the API we want to iterate over all backup groups
belonging to a particular type at least once, and iterating
through *everything* and simply "skipping" over every single
entry from another type makes no sense given that the groups
are organized into subdirectories based on their type.

Let's have an `.iter_backup_type()` method which returns an
iterator over all the groups of a specific type named
ListGroupsType and factorize the type level iterator out of
ListGroups for reuse.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>