xentop: Fix xentop for blktap

xentop: Fix xentop for blktap

Blktap devices information isn't shown by xentop currently.

xen-unstable c/s 17813 said "blktap devices have statistics
counters (e.g., rd_req, wr_req, oo_req) prepended by tap_".
In fact, it is as follows.

# ls -l /sys/devices/xen-backend/tap-1-769/statistics/
total 0
-r--r--r-- 1 root root 4096 Dec  3 20:37 oo_req
-r--r--r-- 1 root root 4096 Dec  3 20:37 rd_req
-r--r--r-- 1 root root 4096 Dec  3 20:37 rd_sect
-r--r--r-- 1 root root 4096 Dec  3 20:37 wr_req
-r--r--r-- 1 root root 4096 Dec  3 20:37 wr_sect

The statistics counters haven't had "tap_" because it was removed
by linux-2.6.18-xen c/s 34.

This patch reverts xen-unstable c/s 17813, then we can get the
blktap devices information by using xentop.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>