experimentally move savevm-async back into a coroutine

experimentally move savevm-async back into a coroutine

Move qemu_savevm_state_{header,setup} into the main loop and
the rest of the iteration into a coroutine. The former need
to lock the iothread (and we can't unlock it in the
coroutine), and the latter can't deal with being in a
separate thread, so a coroutine it must be.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>