xen/blkback: make st_ statistics per ring

xen/blkback: make st_ statistics per ring

Make st_* statistics per ring and the VBD sysfs would iterate over all the
rings.

Note: xenvbd_sysfs_delif() is called in xen_blkbk_remove() before all rings
are torn down, so it's safe.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
v2: Aligned the variables on the same column.
(cherry picked from commit db6fbc106786f26d95889c50c18b1f28aa543a17)
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>