qemu-thread: Add qemu_cond_timedwait

The new function is needed to implement conditional sleep for CPU
throttling. It's possible to reuse qemu_sem_timedwait, but it's more
difficult than just add qemu_cond_timedwait.

Also moved compute_abs_deadline function up the code to reuse it in
qemu_cond_timedwait_impl win32.

Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
Message-Id: <20190909131335.16848-2-yury-kotov@yandex-team.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>