qemu: remove use of (+|-)name syntax for -cpu featres

qemu: remove use of (+|-)name syntax for -cpu featres

The -cpu arg gained support for feature=on|off syntax for the x86
emulator in 2.4.0

  commit 38e5c119c2925812bd441450ab9e5e00fc79e662
  Author: Eduardo Habkost <ehabkost@redhat.com>
  Date:   Mon Mar 23 17:29:32 2015 -0300

    target-i386: Register QOM properties for feature flags

Most other targets gained this syntax even earlier in 1.4.1

  commit 1590bbcb02921dfe8e3cf66e3a3aafd31193babf
  Author: Andreas Färber <afaerber@suse.de>
  Date:   Mon Mar 3 23:33:51 2014 +0100

    cpu: Implement CPUClass::parse_features() for the rest of CPUs

    CPUs who do not provide their own implementation of feature parsing
    will treat each option as a QOM property and set it to the supplied
    value.

There appears no reason to keep supporting "+|-feature" syntax,
given the current minimum QEMU version.

Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>