Apparently on some systems AVIC is disabled in CPUID but still usable.

Allow the user to override the CPUID if the user is willing to
take the risk.
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220301143650.143749-7-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>