Convert qemu-img convert to new bdrv_create

This is part two of the qemu-img conversion. This really works the same as the
previous conversion of qemu-img create: It introduces a new -o option for the
generic approach and adds the old-style options to this option set.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>