KVM: selftests: Do kvm_cpu_has() checks before creating VM+vCPU

Move the AMX test's kvm_cpu_has() checks before creating the VM+vCPU,
there are no dependencies between the two operations.  Opportunistically
add a comment to call out that enabling off-by-default XSAVE-managed
features must be done before KVM_GET_SUPPORTED_CPUID is cached.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221128225735.3291648-5-seanjc@google.com