qdev: convert watchdogs

-watchdog NAME is now equivalent to -device NAME, except it treats
option argument '?' specially, and supports only one watchdog.

A side effect is that a device created with -watchdog may now receive
a different PCI address.

i6300esb is now available on any machine with a PCI bus, not just PCs.
ib700 is still PC only, but that could be changed easily.

The only remaining use of struct WatchdogTimerModel and
watchdog_add_model() is supporting '-watchdog ?'.  Should be replaced
by searching device_info_list for watchdog devices when we can
identify them there.

Also fixes ib700 not to use vm_clock before it is initialized: in
wdt_ib700_init(), called from register_watchdogs(), which runs before
init_timers().  The bug made ib700_write_enable_reg() crash in
qemu_del_timer().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
