block: Generic file creation fallback

If a protocol driver does not support image creation, we can see whether
maybe the file exists already.  If so, just truncating it will be
sufficient.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200122164532.178040-3-mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>