qdict: Add qdict_array_entries()

qdict: Add qdict_array_entries()

This counts the entries in a flattened array in a QDict without
actually splitting the QDict into a QList.

bdrv_open_image() doesn't take a QList, but rather a QDict and a key
prefix string, so this is more convenient for block drivers which have a
dynamically sized list of child nodes (e.g. Quorum) and are to be
converted to using bdrv_open_image() as the standard interface for
opening child nodes.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>