KVM: selftests: Use host page size to map ring buffer in dirty_log_test

In vcpu_map_dirty_ring(), the guest's page size is used to figure out
the offset in the virtual area. It works fine when we have same page
sizes on host and guest. However, it fails when the page sizes on host
and guest are different on arm64, like below error messages indicates.

  # ./dirty_log_test -M dirty-ring -m 7
  Setting log mode to: 'dirty-ring'
  Test iterations: 32, interval: 10 (ms)
  Testing guest mode: PA-bits:40,  VA-bits:48, 64K pages
  guest physical test memory offset: 0xffbffc0000
  vcpu stops because vcpu is kicked out...
  Notifying vcpu to continue
  vcpu continues now.
  ==== Test Assertion Failure ====
  lib/kvm_util.c:1477: addr == MAP_FAILED
  pid=9000 tid=9000 errno=0 - Success
  1  0x0000000000405f5b: vcpu_map_dirty_ring at kvm_util.c:1477
  2  0x0000000000402ebb: dirty_ring_collect_dirty_pages at dirty_log_test.c:349
  3  0x00000000004029b3: log_mode_collect_dirty_pages at dirty_log_test.c:478
  4  (inlined by) run_test at dirty_log_test.c:778
  5  (inlined by) run_test at dirty_log_test.c:691
  6  0x0000000000403a57: for_each_guest_mode at guest_modes.c:105
  7  0x0000000000401ccf: main at dirty_log_test.c:921
  8  0x0000ffffb06ec79b: ?? ??:0
  9  0x0000ffffb06ec86b: ?? ??:0
  10 0x0000000000401def: _start at ??:?
  Dirty ring mapped private

Fix the issue by using host's page size to map the ring buffer.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221110104914.31280-6-gshan@redhat.com