xen: Don't change -drive if=xen device name during machine init

xen: Don't change -drive if=xen device name during machine init

A "top" BlockDriverState has a non-empty device_name.  If the user
doesn't specify one with -drive parameter id, the system supplies a
default name.

xen_config_dev_blk() changes this name, during machine initialization.
Naughty.  Don't do that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>