KVM: selftests: nSVM: Add enlightened MSR-Bitmap selftest

Introduce a new test for Hyper-V nSVM extensions (Hyper-V on KVM) and add
a test for enlightened MSR-Bitmap feature:

- Intercept access to MSR_FS_BASE in L1 and check that this works
  with enlightened MSR-Bitmap disabled.
- Enabled enlightened MSR-Bitmap and check that the intercept still works
  as expected.
- Intercept access to MSR_GS_BASE but don't clear the corresponding bit
  from clean fields mask, KVM is supposed to skip updating MSR-Bitmap02 and
  thus the consequent access to the MSR from L2 will not get intercepted.
- Finally, clear the corresponding bit from clean fields mask and check
  that access to MSR_GS_BASE is now intercepted.

The test works with the assumption, that access to MSR_FS_BASE/MSR_GS_BASE
is not intercepted for L1. If this ever becomes not true the test will
fail as nested_svm_exit_handled_msr() always checks L1's MSR-Bitmap for
L2 irrespective of clean fields. The behavior is correct as enlightened
MSR-Bitmap feature is just an optimization, KVM is not obliged to ignore
updates when the corresponding bit in clean fields stays clear.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220203104620.277031-7-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>