qemu: Simplify interface handling in qemuConnectDomainXMLToNative()
    
    Firstly, VIR_STRDUP() accepts NULL, so there is no need to check
    if the string we want to duplicate is not-NULL. Secondly,
    virDomainNetSetModelString() also accepts NULL. Thirdly, we have
    VIR_AUTOFREE().
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Ján Tomko <jtomko@redhat.com>