[HVM][SVM] Fix 1/2 to interrupt delivery logic.

[HVM][SVM] Fix 1/2 to interrupt delivery logic.

This patch uses the VINTR intercept to signal the hypervisor when a
guest can take an interrupt.  When guest's interrupts are masked by
EFLAGS.IF or the guests are in an interrupt shadow, we create a 'fake'
virtual interrupt to inject while also enabling the VINTR intercept.
When the guest _can_ take interrupts, the hypervisor will #VMEXIT
on VINTR. The VINTR exit handler then clears the VINTR intercept bit
and clears the V_IRQ bit so that svm_intr_assist() can inject a
legitimate interrupt.

Signed-off-by: Travis Betak <travis.betak@amd.com>
Signed-off-by: Wei Huang <wei.huang2@amd.com>
Signed-off-by: Tom Woller <thomas.woller@amd.com>