PCI.ensure_device_frontend_exists: don't overwrite existing frontend keys
PCI.ensure_device_frontend_exists: don't overwrite existing frontend keys

Previously, we were checking for the presence of

    /local/domain/<domid>/device/pci/0backend

to decide whether to write the frontend keys. Instead, check

    /local/domain/<domid>/device/pci/0/backend

In practice, this meant that we were over-writing the frontend keys
whenever ensure_device_frontend_exists was called, including resetting
'state' to 1. This was not the intended behaviour.

Signed-off-by: Jonathan Davies <d3af50858a27f644d377a39d1fbd399608163688@citrix.com>

thomassa/xenopsd,thomassa/xenopsd,thomassa/xenopsd