[CP-9987] & [CA-136792]: Backport of reading blkback stats from tapdisk3

With blktap3, the blkback stats structure is maintained on a shared page
This patch fetches the required counters via C stubs to create the
datasources.

Backport of #xen-api/pull/1917 [b254def49d64679bc09e4f761282eaa758146732]

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
CP-9987: Fix oasis for build

This patch adds the blktap3_stats_stubs to the oasis build environment.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
[CP-9987] & [CA-145335]: Backport of record based model for sysfs stats

This patch introduces a record based model to create statistics.

Backport of #xen-api/pull/1955 [18ecac327bca9b799d4b87710304fdd4f1dd52e6]

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
[CP-9987] & [CA-145335]: Backport of expoting extra tapdisk3 stats

This patch exposes additional statistics exposed by tapdisk3.
Prior to XS6.5, a VM's metrics were supplied by xcp-rrdd,
xcp-rrd-iostat, etc... Originally, these metrics were created by reading
counters from `sysfs`. With tapdisk3, we read the statistics exposed
under '/dev/shm' to create these datasources.
The newly exposed counters are:
* vbd_<X>_iowait
* vbd_<X>_inflight
* vbd_<X>_iops_read
* vbd_<X>_iops_write
* vbd_<X>_iops_total
* vbd_<X>_io_throughput_total

Metrics like vbd_<X>_io_throughput_read and vbd_<X>_io_throughput_write
are redundant.

Backport of #xen-api/pull/1955 [496f14e6a55507d0e2bad2162ed01748d2d52779]

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
[CP-9987] & [CA-147261]: Backport of exposing avgqu_sz for tapdisk3

This patch exposes the average queue size metric for tapdisk3.

Backport of #xen-api/pull/1965 [8bd98706719a031fdb814bd31a1d04e8324599f8]

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>