target-i386: Cleanup 'foo=val' feature handling

Features family, model, stepping, level, hv_spinlocks are treated similarly
when passed from command line, so it's not necessary to handle each of them
individually. Collapse them to one catch-all branch which will treat
any not explicitly handled feature in format 'foo=val'.

Any unknown feature will be rejected by 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>
Signed-off-by: Andreas Färber <afaerber@suse.de>
