libxenstat: Fix statistics for blktap disks on linux

libxenstat: Fix statistics for blktap disks on linux

Overview: update xenstat vbd statistics parsing from
/sys/devices/xen-backend to process blktap disks

Reason:
        -blktap devices (now referenced as tap rather than vbd in
        /sys) have statistics counters
        (e.g.,  rd_req, wr_req, oo_req) prepended by tap_
        -xenstat behavior did not previously account for this
        behavior, which resulted in 0 disks
        visible and similarly impacted disk statistics

To reproduce these conditions, make a domain with tap:aio backed disks
on 3.2.x, run xentop, and press B to view VBD stats (nothing will
appear for the domain using tap:aio)

Signed-off-by: steve.maresca@gmail.com