Allow -1 as the length of "get console output" API

Current "get console output" API does not allow -1 as the length and
returns a BadRequest response if receiving it. The other APIs(quota)
considers -1 as an unlimited value.
This patch allows -1 as the length for API consistency.

DocImpact

Change-Id: Idf88a238d1b0e545ebab5be872269b1b1030cc56
Related-Bug: #1295426
Merge "Allow -1 as the length of "get console output" API"
Merge commit 'refs/changes/38/84138/2' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch