qemu-io: Add discard command

qemu-io: Add discard command

discard [-Cq] off len -- discards a number of bytes at a specified
offset

 discards a range of bytes from the given offset

 Example:
 'discard 512 1k' - discards 1 kilobyte from 512 bytes into the file

 Discards a segment of the currently open file.
 -C, -- report statistics in a machine parsable format
 -q, -- quite mode, do not show I/O statistics

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