migrate: handle storage not selected manually in storage config

migrate: handle storage not selected manually in storage config

e.g. local storage was considered not allowed for offline migration
even if it is available on all nodes, this should now be fixed as it
is now considered available on all nodes if a local storage isn't
restricted to a specific subset of the available nodes. The user is
responseable to make sure that the datacenter storage config reflects
the actual setup, so there is no additional check for local storages
which aren't available on all nodes if they are not explicitly marked
at datacenter level.

Signed-off-by: Tim Marx <t.marx@proxmox.com>