qemu: implement chardev source reconnect
    
    The reconnect attribute for chardev devices in QEMU is used to
    configure the reconnect timeout in seconds.  Setting '0' value disables
    the reconnect functionality thus we don't allow to set '0' for QEMU.
    To disable the reconnect user should use <reconnect enabled='no'/>.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1254971
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>