Introduce allow_rename parameter for pvesm import and storage_migrate

Introduce allow_rename parameter for pvesm import and storage_migrate

and also return the ID of the allocated volume. This option
allows plugins to choose a new name if there is a collision.

In storage_migrate, the API version of the receiving side is checked.

In Storage.pm's volume_import, when a plugin returns 'undef',
it can be assumed that the import with the requested volid was
successful (it should've died otherwise) and so volid is returned.
This is done for backwards compatibility with foreign plugins.

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