SR: Only consider a filename to be a UUID if it is the correct length

Uuid.is_uuid only checks that the string passed to it _starts_ with a
valid UUID, so Uuid.is_uuid "82cf569a-344d-9c62-95bc-ac693202e25a_foo_bar"
returns True.   Uuidm's behaviour is the same...

Check that the file's base name is the right length, in addition to parsing
correctly.

Signed-off-by: Euan Harris <euan.harris@citrix.com>