KVM: selftests: Cache CPUID in struct kvm_vcpu

Cache a vCPU's CPUID information in "struct kvm_vcpu" to allow fixing the
mess where tests, often unknowingly, modify the global/static "cpuid"
allocated by kvm_get_supported_cpuid().

Add vcpu_init_cpuid() to handle stuffing an entirely different CPUID
model, e.g. during vCPU creation or when switching to the Hyper-V enabled
CPUID model.  Automatically refresh the cache on vcpu_set_cpuid() so that
any adjustments made by KVM are always reflected in the cache.  Drop
vcpu_get_cpuid() entirely to force tests to use the cache, and to allow
adding e.g. vcpu_get_cpuid_entry() in the future without creating a
conflicting set of APIs where vcpu_get_cpuid() does KVM_GET_CPUID2, but
vcpu_get_cpuid_entry() does not.

Opportunistically convert the VMX nested state test and KVM PV test to
manipulating the vCPU's CPUID (because it's easy), but use
vcpu_init_cpuid() for the Hyper-V features test and "emulator error" test
to effectively retain their current behavior as they're less trivial to
convert.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-19-seanjc@google.com