qemu-img: Eliminate bdrv_new_open() code duplication

qemu-img: Eliminate bdrv_new_open() code duplication

Several commands have code to create a BlockDriverState and open a file.
The bdrv_new_open() function can be used to perform these steps.  This
patch converts the qemu-img commands to actually use bdrv_new_open().

Replaced the bdrv_new_open() 'readonly' argument with bdrv_open()-style
flags to support generic flags like BDRV_O_NO_BACKING.

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