Before this, a storage driver would be defaulted to based on the
priority list, and only print a warning if there is state from other
drivers.
This meant a reordering of priority list would "break" users in an
upgrade of docker, such that there images in the prior driver's state
were now invisible.
With this change, prior state is scanned, and if present that driver is
preferred.
As such, we can reorder the priority list, and after an upgrade,
existing installs with prior drivers can have a contiguous experience,
while fresh installs may default to a driver in the new priority list.
Ref: https://github.com/docker/docker/pull/11962#issuecomment-88274858
Signed-off-by: Vincent Batts <vbatts@redhat.com>
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