Some emulated disk operations (MODE SELECT, UNMAP, WRITE SAME)
can trigger asynchronous I/Os.  Provide the cancel_io callback
to ensure that AIOCBs are properly cleaned up.
Signed-off-by: NEric Farman <farman@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
[Tweak commit message. - Paolo]
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>