SVM: Add union intstat_t for offset 68h in vmcb struct

SVM: Add union intstat_t for offset 68h in vmcb struct

According to chapter "Appendix B Layout of VMCB" in the new version
(v3.32) AMD64 APM[1], bit 1 of the VMCB offset 68h is defined as
GUEST_INTERRUPT_MASK.

In current xen codes, it use whole u64 interrupt_shadow to setup
interrupt shadow, which will misuse other bit in VMCB offset 68h
as part of interrupt_shadow, causing svm_get_interrupt_shadow() to
mistake the guest having interrupts enabled as being in an interrupt
shadow.  This has been observed to cause SeaBIOS to hang on boot.

Add union intstat_t for VMCB offset 68h and fix codes to only use
bit 0 as intr_shadow according to the new APM description.

Reference:
[1] https://www.amd.com/system/files/TechDocs/24593.pdf

Signed-off-by: Pu Wen <puwen@hygon.cn>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>