qemu-io: add "abort" command to simulate program crash

Avoiding data loss and corruption is the top requirement for image file
formats.  The qemu-io "abort" command makes it possible to simulate
program crashes and does not give the image format a chance to cleanly
shut down.  This command is useful for data integrity test cases.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
