target-i386: Merge feature filtering/checking functions

Merge filter_features_for_kvm() and kvm_check_features_against_host().

Both functions made exactly the same calculations, the only difference
was that filter_features_for_kvm() changed the bits on cpu->features[],
and kvm_check_features_against_host() did error reporting.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
