qemu: Update guest CPU def in live XML

qemu: Update guest CPU def in live XML

Storing the updated CPU definition in the live domain definition saves
us from having to update it over and over when we need it. Not to
mention that we will soon further update the CPU definition according to
QEMU once it's started.

A highly wanted side effect of this patch, libvirt will pass all CPU
features explicitly specified in domain XML to QEMU, even those that are
already included in the host model.

This patch should fix the following bugs:
    https://bugzilla.redhat.com/show_bug.cgi?id=1207095
    https://bugzilla.redhat.com/show_bug.cgi?id=1339680
    https://bugzilla.redhat.com/show_bug.cgi?id=1371039
    https://bugzilla.redhat.com/show_bug.cgi?id=1373849
    https://bugzilla.redhat.com/show_bug.cgi?id=1375524
    https://bugzilla.redhat.com/show_bug.cgi?id=1377913

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>