verified on qemu-kvm-0.12.1.2-2.90.el6
run guest with -M (rhel6.0.0|rhel5.5.0|rhel5.4.4|rhel5.4.0)
dmidecode (-t 1) in rhel6 guest is :
Manufacturer: Red Hat
Product Name: KVM
Version: RHEL 6.0.0 PC
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Manufacturer: Red Hat
Product Name: KVM
Version: RHEL 5.5.0 PC
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Manufacturer: Red Hat
Product Name: KVM
Version: RHEL 5.4.4 PC
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Manufacturer: Red Hat
Product Name: KVM
Version: RHEL 5.4.0 PC