Xapi: Metadata injection attack against backup/restore functionality

For a brief summary of Xapi terminology, see:

https://xapi-project.github.io/xen-api/overview.html#object-model-overview

Xapi contains functionality to backup and restore metadata about Virtual
Machines and Storage Repositories (SRs).

The metadata itself is stored in a Virtual Disk Image (VDI) inside an
SR. This is used for two purposes; a general backup of metadata
(e.g. to recover from a host failure if the filer is still good), and
Portable SRs (e.g. using an external hard drive to move VMs to another
host).

Metadata is only restored as an explicit administrator action, but
occurs in cases where the host has no information about the SR, and must
locate the metadata VDI in order to retrieve the metadata.

The metadata VDI is located by searching (in UUID alphanumeric order)
each VDI, mounting it, and seeing if there is a suitable metadata file
present. The first matching VDI is deemed to be the metadata VDI, and
is restored from.

In the general case, the content of VDIs are controlled by the VM owner,
and should not be trusted by the host administrator.

A malicious guest can manipulate its disk to appear to be a metadata
backup.

A guest cannot choose the UUIDs of its VDIs, but a guest with one disk
has a 50% chance of sorting ahead of the legitimate metadata backup. A
guest with two disks has a 75% chance, etc.