xend: Rename device backend value when xm save/migrate

xend: Rename device backend value when xm save/migrate

The Xend has a problem that it often fails to restore/migrate
a PV domain whose device backends are partly a driver domain.

Because a checkpoint of the PV domain has device backend value as
domain id, you can restore/migrate the PV domain only when a driver
domain is the same id as device backend value in the checkpoint.

I attached a patch to fix it by renaming device backend value in a
checkpoint from domain id to domain name when xm save/migrate.

This patch doesn't rename device backend value if the value is 0,
which is Domain-0, so the checkpoint format is compatible if you use
only Domain-0 as device backend.

Signed-off-by: Rikiya Ayukawa <ayukawa.rikiya@jp.fujitsu.com>