kvm-unit-test: nVMX: Test GUEST_DR7 on vmentry of nested guests

According to section "Checks on Guest Control Registers, Debug Registers, and
and MSRs" in Intel SDM vol 3C, the following checks are performed on vmentry
of nested guests:

   If the "load debug controls" VM-entry control is 1,

      - bits 63:32 in the DR7 field must be 0.

Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Reviewed-by: Karl Heubaum <karl.heubaum@oracle.com>
Co-developed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>