TLReplaceDisks: Add check if disks are activated

Previously we failed later with a rather useless error message. This
patch fixes this and tells the user to activate-disks if replace-disks
is in the need of activated disks rather than abort with a cryptic error
message.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>