qemu: Don't update CPU when formatting live def
    
    Since commit v2.2.0-199-g7ce711a30e libvirt stores an updated guest CPU
    in domain's live definition and there's no need to update it every time
    we want to format the definition. The commit itself tried to address
    this in qemuDomainFormatXML, but forgot to fix qemuDomainDefFormatLive.
    Not to mention that masking a previously set flag is only acceptable if
    the flag was set by a public API user. Internally, libvirt should have
    never set the flag in the first place.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1485022
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>