CA-146911: Upgrade device names on import to xenopsd

When pushing VM metadata to xenopsd, xapi now uses xvd as the prefix for
HVM as well as PV VBD device names.

VMs migrating from XS 6.2 or earlier will however be using hd as the prefix -
we must upgrade these device names to use xvd on import, otherwise xapi will
push conflicting data to xenopsd, causing xenopsd to attempt to remove the hd*
devices (which will fail if they are attached).