hw/i386/amd_iommu: Simplify non-KVM checks on XTSup feature

hw/i386/amd_iommu: Simplify non-KVM checks on XTSup feature

Generic code wanting to access KVM specific methods should
do so being protected by the 'kvm_enabled()' helper.

Doing so avoid link failures when optimization is disabled
(using --enable-debug), see for example commits c04cfb4596a
("hw/i386: fix short-circuit logic with non-optimizing builds")
and 0266aef8cd6 ("amd_iommu: Fix kvm_enable_x2apic link error
with clang in non-KVM builds").

XTSup feature depends on KVM, so protect the whole block
checking the XTSup feature with a check on whether KVM is
enabled.

Since x86_cpus_init() already checks APIC ID > 255 imply
kernel support for irqchip and X2APIC, remove the confuse
and unlikely reachable "AMD IOMMU xtsup=on requires support
on the KVM side" message.

Fix a type in "configuration" in error message.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Message-Id: <20241129155802.35534-1-philmd@linaro.org>