qemu-timer: make qemu_timer_mod_ns() and qemu_timer_del() thread-safe

Introduce QEMUTimerList->active_timers_lock to protect the linked list
of active timers.  This allows qemu_timer_mod_ns() to be called from any
thread.

Note that vm_clock is not thread-safe and its use of
qemu_clock_has_timers() works fine today but is also not thread-safe.

The purpose of this patch is to eventually let device models set or
cancel timers from a vcpu thread without holding the global mutex.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
