qemu-img bench

This adds a qemu-img command that allows doing some simple benchmarks
for the block layer without involving guest devices and a real VM.

For the start, this implements only a test of sequential reads.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>