Paravirtualized features have been listed in KVM_GET_SUPPORTED_CPUID since
Linux 2.6.35 (commit 84478c829d0f, "KVM: x86: export paravirtual cpuid flags
in KVM_GET_SUPPORTED_CPUID", 2010-05-19).  It has been more than 10 years,
so remove the fallback code.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>