CA-142270: Include blktap3 perf data to host level stats

Host level metrics were accumulated from the VDI level stats. Prior XS
6.5, the VDI level stats were read from sysfs. With blktap3, stats
exposed under /dev/shm should also be taken into account.

Signed-off-by: Kaifeng Zhu <kaifeng.zhu@citrix.com>