util: Add coroutine backend for emscripten

util: Add coroutine backend for emscripten

Emscripten does not support couroutine methods currently used by QEMU but
provides a coroutine implementation called "fiber". This commit introduces a
coroutine backend using fiber. Note that fiber does not support submitting
coroutines to other threads.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/006b683fd578ed6303a2dc8679094da9a7e6dfb4.1745820062.git.ktokunaga.mail@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>