qemu/timer: Clarify timer_new*() must be freed with timer_free()

qemu/timer: Clarify timer_new*() must be freed with timer_free()

There was not mention QEMUTimer created with timer_new*() must
be released with timer_free() instead of g_free(), because then
active timers are removed from the active list. Update the
documentation mentioning timer_free().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>