coroutine: Move coroutine_fn to qemu/osdep.h, trim includes

coroutine: Move coroutine_fn to qemu/osdep.h, trim includes

block/block-hmp-cmds.h and qemu/co-shared-resource.h use coroutine_fn
without including qemu/coroutine.h.  They compile only if it's already
included from elsewhere.

I could fix that, but pulling in qemu/coroutine.h and everything it
includes just for a macro that expands into nothing feels silly.
Instead, move the macro to qemu/osdep.h.

Inclusions of qemu/coroutine.h just for coroutine_fn become
superfluous.  Drop them.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221221131435.3851212-3-armbru@redhat.com>