aio: Another fix to the walking_handlers logic

The AIO dispatch loop will call QLIST_REMOVE and g_free even if there
are other pending calls to qemu_aio_wait outside the current one.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
