mshv: Add debugfs to view hypervisor statistics

mshv: Add debugfs to view hypervisor statistics

Introduce a debugfs interface to expose root and child partition stats
when running with mshv_root.

Create a debugfs directory "mshv" containing 'stats' files organized by
type and id. A stats file contains a number of counters depending on
its type. e.g. an excerpt from a VP stats file:

TotalRunTime                  : 1997602722
HypervisorRunTime             : 649671371
RemoteNodeRunTime             : 0
NormalizedRunTime             : 1997602721
IdealCpu                      : 0
HypercallsCount               : 1708169
HypercallsTime                : 111914774
PageInvalidationsCount        : 0
PageInvalidationsTime         : 0

On a root partition with some active child partitions, the entire
directory structure may look like:

mshv/
  stats             # hypervisor stats
  lp/               # logical processors
    0/              # LP id
      stats         # LP 0 stats
    1/
    2/
    3/
  partition/        # partition stats
    1/              # root partition id
      stats         # root partition stats
      vp/           # root virtual processors
        0/          # root VP id
          stats     # root VP 0 stats
        1/
        2/
        3/
    42/             # child partition id
      stats         # child partition stats
      vp/           # child VPs
        0/          # child VP id
          stats     # child VP 0 stats
        1/
    43/
    55/

On L1VH, some stats are not present as it does not own the hardware
like the root partition does:
- The hypervisor and lp stats are not present
- L1VH's partition directory is named "self" because it can't get its
  own id
- Some of L1VH's partition and VP stats fields are not populated, because
  it can't map its own HV_STATS_AREA_PARENT page.

Co-developed-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Signed-off-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Co-developed-by: Praveen K Paladugu <prapal@linux.microsoft.com>
Signed-off-by: Praveen K Paladugu <prapal@linux.microsoft.com>
Co-developed-by: Mukesh Rathor <mrathor@linux.microsoft.com>
Signed-off-by: Mukesh Rathor <mrathor@linux.microsoft.com>
Co-developed-by: Purna Pavan Chandra Aekkaladevi <paekkaladevi@linux.microsoft.com>
Signed-off-by: Purna Pavan Chandra Aekkaladevi <paekkaladevi@linux.microsoft.com>
Co-developed-by: Jinank Jain <jinankjain@microsoft.com>
Signed-off-by: Jinank Jain <jinankjain@microsoft.com>
Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Reviewed-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Acked-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>