qemu-img: Add cache command line option

qemu-img currently writes disk images using writeback and filling
up the cache buffers which are then flushed by the kernel preventing
other processes from accessing the storage.
This is particularly bad in cluster environments where time-based
algorithms might be in place and accessing the storage within
certain timeouts is critical.
This patch adds the option to choose a cache method when writing
disk images.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
