build: move coroutine backend selection to meson

To simplify the code, rename coroutine-win32.c to match the option
passed to configure.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

build: move coroutine backend selection to meson

To simplify the code, rename coroutine-win32.c to match the option
passed to configure.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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