kvm-unit-tests: nVMX: Test GUEST_LIMIT_GDTR and GUEST_LIMIT_IDTR on vmentry of nested guests

According to section "Checks on Guest Descriptor-Table Registers" in Intel
SDM vol 3C, the following checks are performed on the Guest Descriptor-Table
Registers on vmentry of nested guests:

    - Bits 31:16 of each limit field must be 0.

Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Message-Id: <20200523002603.32450-4-krish.sadhukhan@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>