qemu-iotests: fix 028 failure due to disk image path

qemu-iotests: fix 028 failure due to disk image path

The disk image path is echoed by QEMU's readline when the "drive_backup
disk ${TEST_IMG}.copy" HMP command is issued.  Unfortunately it is very
hard to filter out the path due to readline's character-by-character
output (with terminal escape sequences).  Just redirect this command to
/dev/null for now.

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