xen-blkfront: free resources if xlvbd_alloc_gendisk fails

Current code forgets to free resources in the failure path of
xlvbd_alloc_gendisk(), this patch fix it.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>