vl.c: remove init_clocks call from main

Clocks are initialized in qemu_init_main_loop. They are not needed before it.
Initializing them twice is not only unnecessary but is harmful: it results in
memory leak and potentially can lead to a situation where different parts of
QEMU use different sets of timers.

To avoid it remove init_clocks call from main and add an assertion to
qemu_clock_init that corresponding clock has not been initialized yet.

Signed-off-by: Kirill Batuzov <batuzovk@ispras.ru>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
