Ensure watchdog remains disabled unless enabled on the command line.

Ensure watchdog remains disabled unless enabled on the command line.

Bail out of setup_apic_nmi_watchdog() early if watchdog is not
enabled. Is the watchdog is disabled then increment
watchdog_disable_count to prevent it ever becoming enabled.

Unconditionally call setup_apic_nmi_watchdog() from setup_local_APIC()
so that the above is guaranteed to have occurred before the first call
of watchdog_enable().

Signed-off-by: Ian Campbell <Ian.Campbell@XenSource.com>