qapi-gen: mark coroutine QMP command functions as coroutine_fn

qapi-gen: mark coroutine QMP command functions as coroutine_fn

Coroutine commands have to be declared as coroutine_fn, but the
marker does not show up in the qapi-comands-* headers; likewise, the
marshaling function calls the command and therefore must be coroutine_fn.
Static analysis would want coroutine_fn to match between prototype and
declaration, because in principle coroutines might be compiled to a
completely different calling convention.  So we would like to add the
marker to the header.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>