qemu: move virDomainLiveConfigHelperMethod right after BeginJob
    
    We should call virDomainLiveConfigHelperMethod ASAP because this
    function transfers VIR_DOMAIN_AFFECT_CURRENT to VIR_DOMAIN_AFFECT_LIVE
    or VIR_DOMAIN_AFFECT_CONFIG.  All other additional checks for those two
    flags should consider that the user give us VIR_DOMAIN_AFFECT_CURRENT.
    
    Remove the unnecessary check whether the domain is live in case of
    VIR_DOMAIN_VCPU_GUEST because this check is done by
    virDomainLiveConfigHelperMethod.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>