kvm: x86: svm: Add support to (de)activate posted interrupts

Introduce interface for (de)activate posted interrupts, and
implement SVM hooks to toggle AMD IOMMU guest virtual APIC mode.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>