x86: VMX: Verify do not lose pending interrupt queued before entering guest

This patch test the issue fixed by KVM commit "KVM: nVMX: Fix loss of
pending event before entering L2". The test aim to verify that a pending
interrupt while interrupts are disabled is dispatched when we enter into
VMX guest instead of being lost.

The test configures VMCS to intercept external-interrupts and then
queue an interrupt by disabling interrupts and issue a self-IPI.
At this point, we enter guest and we expect CPU to immediately exit
guest on external-interrupt. To complete the test, we then re-enable
interrupts, verify interrupt is dispatched and re-enter guest to verify
it completes execution.

Reviewed-by: Nikita Leshchenko <nikita.leshchenko@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>