qemu: Don't update CPU when formatting live def

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>