xen: use qdev_unplug() instead of g_free() in xen_pv_find_xendev()

The error exits of xen_pv_find_xendev() free the new xen-device via
g_free() which is wrong.

As the xen-device has been initialized as qdev it must be removed
via qdev_unplug().

This bug has been introduced with commit 3a6c9172ac5951e6dac2b3f6
("xen: create qdev for each backend device").

Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Tested-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
