cpu_conf: Fix default value for CPU match attribute

cpu_conf: Fix default value for CPU match attribute

Commit v0.8.4-66-g95ff6b18ec (9 years ago) changed the default value for
the cpu/@match attribute to 'exact' in a rather complicated way. It did
so only if <model> subelement was present and set -1 otherwise (which is
not expected to ever happen). Thus the following two equivalent XML
elements:

    <cpu mode='host-model'/>

and

    <cpu mode='host-model'>
      <model/>
    </cpu>

would be parsed differently. The former would end up with match == -1
while the latter would have match == 1 ('exact'). This is not a big deal
since the match attribute is ignored for host-model CPUs, but we can
simplify the code and make it a little bit saner anyway.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>