Implement query-cpu-model-expansion for target-i386.

This should meet all the requirements while being simple. In the
case of static expansion, it will use the new "base" CPU model,
and in the case of full expansion, it will keep the original CPU
model name+props, and append extra properties.

A future follow-up should improve the implementation of
type=full, so that it returns more detailed data, including every
writable QOM property in the CPU object.

Cc: libvir-list@redhat.com
Cc: Jiri Denemark <jdenemar@redhat.com>
Message-Id: <20170222190029.17243-3-ehabkost@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>