initialize struct sigevent before timer_create

When qemu is run under valgrind, valgrind shows the following output
on exit:

==3648== 1 errors in context 2 of 2:
==3648== Syscall param timer_create(evp) points to uninitialised byte(s)
==3648==    at 0x54E936A: timer_create (in /lib/librt-2.9.so)
==3648==    by 0x405DCF: dynticks_start_timer (vl.c:1549)
==3648==    by 0x40A966: main (vl.c:1726)
==3648==  Address 0x7fefffb34 is on thread 1's stack
==3648==  Uninitialised value was created by a stack allocation
==3648==    at 0x405D60: dynticks_start_timer (vl.c:1534)

This patch is a simple fix to remove this potential problem.

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
