Fix the "hda lost interrupt" issue when creating a VMX guest on a PAE

Fix the "hda lost interrupt" issue when creating a VMX guest on a PAE
host.

Occasionally when injecting an IDE DMA interrupt into the guest, a
page fault occurs (e.g., because the IDT mapping is not present in
shadow pagetables).  This causes an immediate vmexit and, because it
occurred during event delivery, the original VM_ENTRY_INTR_INFO_FIELD
is kept in IDT_VECTORING_INFO_FIELD.

The current code copies IDT_VECTORING_INFO_FIELD back to
VM_ENTRY_INTR_INFO_FIELD, intending that the interrupt will be
injected again on next vmresume.

However, there is a corner case: if, before the next vmresume, a timer
interrupt happened then vmx_intr_assist may overwrite the information
on VM_ENTRY_INTR_INFO_FIELD, and the IDE DMA interrupt is effectively
lost.

This patch checks the IDT_VECTORING_INFO_FIELD in vmx_intr_assist and,
if it is set, copies it to VM_ENTRY_INTR_INFO_FIELD and returns.

Signed-off-by: Yunhong Jiang <Yunhong.jiang@intel.com>
Signed-off-by: Eddie Dong <eddie.dong@intel.com>