diskmanage: allow passing partitions to get_disks

diskmanage: allow passing partitions to get_disks

Requires that the $include_partitions parameter is set too, which:
1. Makes sense, because the partition won't be included in the result
   otherwise.
2. Ensures backwards compatibility for existing callers that don't
   use $include_partitions. No existing callers use both $disks and
   $include_partitions at the same time, so nothing learns to
   "support" partitions by accident.

Moving the strip_dev helper to the top, so it can be used everywhere.

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