Since VIR_DOMAIN_AFFECT_{LIVE,CONFIG,CURRENT} was created,
all new virsh commands use "--config" to represents the
persistent changing. This patch add "--config" option
for the old commands which still use "--persistent",
and "--persistent" is now alias of "--config".

tools/virsh.c: (use "--config", and "--persistent" is
    alias of "--config" now).
    cmdDomIfSetLink, cmdDomIfGetLink, cmdAttachDevice,
    cmdDetachDevice, cmdUpdateDevice, cmdAttachInterface,
    cmdDetachInterface, cmdAttachDisk, cmdDetachDisk

toos/virsh.pod: Update docs of the changed commands, and
    add some missed docs for "--config" (detach-interface,
    detach-disk, and detach-device).