Although we have long supported 'qemu-img convert -o
backing_file=foo,backing_fmt=bar', the fact that we have a shortcut -B
for backing_file but none for backing_fmt has made it more likely that
users accidentally run into:
qemu-img: warning: Deprecated use of backing file without explicit backing
format
when using -B instead of -o. For similarity with other qemu-img
commands, such as create and compare, add '-F $fmt' as the shorthand
for '-o backing_fmt=$fmt'. Update iotest 122 for coverage of both
spellings.
Signed-off-by: Eric Blake<eblake@redhat.com>