Commit 2ace7a87 introduced a logic bug by an improperly
modified condition where we'd skip to the else branch when reusing of
external images was requested and blockdev is available.

The original intentions were to skip the backing store update with
blockdev.

Fix it by only asserting the boolean which was used to track whether we
support update of the backing store only when blockdev is not present
along with the appropriate rename.

https://bugzilla.redhat.com/show_bug.cgi?id=1820016Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>