libxl: add support for migrating HVM guests without a device model

libxl: add support for migrating HVM guests without a device model

Only some minor libxl changes are needed in order to be able to migrate HVM
guests without a device model, no hypervisor changes are needed.

This change prevents sending the emulator context if the device model
version is set to none.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>