migration: fail when aliased volume is detected

migration: fail when aliased volume is detected

Aliased volids can lead to unexpected behavior in a migration.

An aliased volid can happen if we have two storage configurations,
pointing to the same place. The resulting 'path' for a disk image
will be the same.
Therefore, stop the migration in such a case.

The check works by comparing the path returned by the storage plugin.

We decided against checking the storages themselves being aliased. It is
not possible to infer that reliably from just the storage configuration
options alone.

Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>