add Storage::get_bandwidth_limit helper

add Storage::get_bandwidth_limit helper

Takes an operation, an optional requested bandwidth
limit override, and a list of storages involved in the
operation and lowers the requested bandwidth against global
and storage-specific limits unless the user has permissions
to change those.
This means:
 * Global limits apply to all users without Sys.Modify on /
   (as they can change datacenter.cfg options via the API).
 * Storage specific limits apply to users without
   Datastore.Allocate access on /storage/X for any involved
   storage X.

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