Allow importing VM metadata to a different SR than the original source

The exported metadata file for a VM (xe vm-export uuid=XYZ metadata=true) along with the full VM export (XVA file) are created. If these are copied either to a different SR (because the original storage failed) or to another pool, and the metadata file is imported to a different SR, it fails with "Error code: VDI_CONTENT_ID_MISSING". This makes the VM metadata useless in the event of a disaster recovery or the desire to import the backed up VM to a different pool or even different SR than from where it was originally exported.

There should be a way to override the original SR restriction and make it possible to import the VM along with its metadata to any SR, even if some of the original information cannot be preserved. In particular, if the original storage SR is destroyed, it can never be re-created with the same UUID, hence the VM metadata export file can never be imported back onto an SR with a different UUID than the one from which it was exported from under the current restrictions.