migrate: use correct target storage id for checks

migrate: use correct target storage id for checks

The '--targetstorage' parameter does not apply to shared storages.

Example for a problem solved with the enabled check: Given a VM with
images only on a shared storage 'storeA', not available on the target
node (i.e. restricted by the nodes property). Then using
'--targetstorage storeB' would make offline migration suddenly
"work", but of course the disks would not be accessible and then
trying to migrate back would fail...

Example for a problem solved with the content type check: if a
VM had a shared ISO image, and there was a '--targetstorage storeA'
option, availablity of the 'iso' content type is checked for
'storeA', which is wrong as the ISO would not be moved to that
storage.

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