now that all accelerators support the CpusAccel interface,
we can remove most checks for non-NULL cpus_accel,
we just add a sanity check/assert at vcpu creation.

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>