target-i386: Cleanup 'foo' feature handling

Features check, enforce, hv_relaxed and hv_vapic are treated as boolean
set to 'on' when passed from command line, so it's not necessary to
handle each of them separately. Collapse them to one catch-all branch
which will treat any feature in format 'foo' as boolean set to 'on'.

Any unknown feature will be rejected by CPU property setter so there is no
need to check for unknown feature in cpu_x86_parse_featurestr(), therefore
it's replaced by above mentioned catch-all handler.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
