bash-completion: Add fallback for symlinks/images

bash-completion: Add fallback for symlinks/images

For commands which support operating on files (i.e. disk images), it is
desirable for bash-completion to complete matching file names.  It is
also desirable to complete on block device symlinks (e.g. under
/dev/disk).  To complete common use cases, often on canonical device
names, continue to try completion using canonical device names, then
fall back to matching any file incrementally as Bash does by default.[1]

[1]: https://github.com/karelzak/util-linux/issues/842#issuecomment-523450243

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>