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

Introduce a test for enlightened MSR-Bitmap feature (Hyper-V on KVM):
- 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 'hv_clean_fields', 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 'hv_clean_fields' 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_vmx_exit_handled_msr() always checks L1's MSR-Bitmap for
L2 irrespective of 'hv_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 'hv_clean_fields' stays
clear.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220203104620.277031-4-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>