KVM: selftests: Extend state_test to check vGIF

KVM: selftests: Extend state_test to check vGIF

V_GIF_MASK is one of the fields written by the CPU after VMRUN, and
sync'd by KVM from vmcb02 to cached vmcb12 after running L2. Part of the
reason is to make sure V_GIF_MASK is saved/restored correctly, as the
cached vmcb12 is the payload of nested state.

Verify that V_GIF_MASK is saved/restored correctly in state_test by
enabling vGIF in vmcb12, toggling GIF in L2 at different GUEST_SYNC()
points, and verifying that V_GIF_MASK is correctly propagated to the
nested state.

Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260225005950.3739782-4-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>