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>
Message-Id: <20210913131735.1948339-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>