qemu: always translate underscores to hyphens in CPU features

qemu: always translate underscores to hyphens in CPU features

QEMU switched from using underscores in x86 CPU features to hyphens
in the 2.8.0 series with two commits

  commit fc7dfd205f3287893c436d932a167bffa30579c8 (HEAD, refs/bisect/bad)
  Author: Eduardo Habkost <ehabkost@redhat.com>
  Date:   Fri Sep 30 15:49:40 2016 -0300

    target-i386: Remove underscores from feat_names arrays

  commit 54b8dc7c19cd781e96f1e9b001ca6001d804eb19
  Author: Eduardo Habkost <ehabkost@redhat.com>
  Date:   Fri Sep 30 15:49:38 2016 -0300

    target-i386: Register aliases for feature names with underscores

Libvirt names use underscores so we conditionally tranlate the
names when talking to new QEMU. Since the min QEMU was raised to
version 2.11.0, all QEMU versions we talk to expect hypens, so
the translation can be done unconditionally.

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