x86: Add a regression test for L1 LDTR persistence bug

Add a regression test for Linux commit afc8de0118be ("KVM: nVMX: Set
LDTR to its architecturally defined value on nested VM-Exit"). L1's
LDTR should be 0 after an emulated VM-exit from L2.

Signed-off-by: Jim Mattson <jmattson@google.com>
Message-Id: <20211015195530.301237-4-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>