I'm not really certain what they're referring to. The VM itself should be completely portable between VMMs. The VMMs, however, are completely system/processor dependent.
MSRs and the like should not pose an issue for the VM as the VMM should present Intel or AMD compatible via the CPUID instruction. If the OS supports AMD processors natively then it should support the AMD CPUID. Similarly, if the OS supports Intel it should support Intel CPUID.
Edit: OK, I missed the part about Live Migration. I never really saw this as an issue because I never considered using Live Migration in a mixed-processor environment. Generally, I wouldn't recommend it (I highly doubt that AMD and Intel are suddenly going to become identical).
MSRs and the like should not pose an issue for the VM as the VMM should present Intel or AMD compatible via the CPUID instruction. If the OS supports AMD processors natively then it should support the AMD CPUID. Similarly, if the OS supports Intel it should support Intel CPUID.
Edit: OK, I missed the part about Live Migration. I never really saw this as an issue because I never considered using Live Migration in a mixed-processor environment. Generally, I wouldn't recommend it (I highly doubt that AMD and Intel are suddenly going to become identical).