block: Add BlockDriver.is_format

block: Add BlockDriver.is_format

We want to unify child_format and child_file at some point.  One of the
important things that set format drivers apart from other drivers is
that they do not expect other format nodes under them (except in the
backing chain), i.e. we must not probe formats inside of formats.  That
means we need something on which to distinguish format drivers from
others, and hence this flag.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20200513110544.176672-3-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>