KVM: SVM: Modify AVIC GATag to support max number of 512 vCPUs

Define AVIC_VCPU_ID_MASK based on AVIC_PHYSICAL_MAX_INDEX, i.e. the mask
that effectively controls the largest guest physical APIC ID supported by
x2AVIC, instead of hardcoding the number of bits to 8 (and the number of
VM bits to 24).

The AVIC GATag is programmed into the AMD IOMMU IRTE to provide a
reference back to KVM in case the IOMMU cannot inject an interrupt into a
non-running vCPU.  In such a case, the IOMMU notifies software by creating
a GALog entry with the corresponded GATag, and KVM then uses the GATag to
find the correct VM+vCPU to kick.  Dropping bit 8 from the GATag results
in kicking the wrong vCPU when targeting vCPUs with x2APIC ID > 255.

Fixes: 4d1d7942 ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
Cc: stable@vger.kernel.org
Reported-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Tested-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Message-Id: <20230207002156.521736-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>