IgvmParamBlock.is_qemu field is ambiguous

[jxu023]

In the fashion of platform: suppress SVSM use of interrupts on KVM/QEMU we'd also like to disable SVSM interrupts on vanadium.

However, the is_qemu field used as a hypervisor check is also used to signify the presence of qemu devices like FwCfg and TestDev. These may also available on Vanadium.

Thus, to disambiguate is_qemu and provide a more general hypervisor check, let's add a hypervisor field to IgvmParamBlock, and perhaps rename is_qemu to something else. Then we can configure the use of TestDev with SVSM on Vanadium and run SVSM with suppressed interrupts on Vanadium.