Fix assertion failure during many-cpu boot

While booting with many cpus (e.g., run.py -c 20, but I sometimes saw this
with as few as 7), we crashed while trying to sleep with preemption disabled:

Assertion failed: preemptable() (/home/nyh/osv/include/sched.hh: do_wait_until: 605)
Aborted

Turns out that since commit 223b2252815f22 (half a year ago), main_cont()
ran smp_launch() with preemption disabled. But smp_launch creates threads,
and thread's constructor may sleep (e.g., on the thread_list_mutex), and
we cannot do this with preemption disabled...

So dropped the preempt_disable()/enable() from main_cont(). The reasoning for
it as expressed in 223b2252815f22 appears to be no longer relevant.

Fixes #130.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>