hw/i386/amd_iommu: Fix xtsup when vcpus < 255

hw/i386/amd_iommu: Fix xtsup when vcpus < 255

If vCPUs > 255 then x86 common code (x86_cpus_init()) call kvm_enable_x2apic().
But if vCPUs <= 255 then the common code won't calls kvm_enable_x2apic().

This is because commit 8c6619f3e692 ("hw/i386/amd_iommu: Simplify non-KVM
checks on XTSup feature") removed the call to kvm_enable_x2apic when xtsup
is "on", which break things when guest is booted with x2apic mode and
there are <= 255 vCPUs.

Fix this by adding back kvm_enable_x2apic() call when xtsup=on.

Fixes: 8c6619f3e692 ("hw/i386/amd_iommu: Simplify non-KVM checks on XTSup feature")
Reported-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Tested-by: Tested-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
Message-Id: <20250516100535.4980-3-sarunkod@amd.com>
Fixes: 8c6619f3e692 ("hw/i386/amd_iommu: Simplify non-KVM checks on XTSup feature")
Reported-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Tested-by: Tested-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
(cherry picked from commit 0f178860df3489a9d3c19a5f7f024e6aa6c26515)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>