Pop out 'offset' and 'limit' before use for filter

Pop out 'offset' and 'limit' before use for filter

In previous code of _items() from api/v{1,2}/snapshots.py,
and also the _items)_ from api/v1/volume.py.we didn't pop
out the 'offset' and 'limit' fields from HTTP get params
before we use such params for filter.This is the root cause
for bug #1205956

For non-admin user, since 'offset' and 'limit' is not in the
allowed_search_options, so the volumes.remove_invalid_options
will help to filter them out. As a result, it walks around this
bug.

But for admin user,the volumes.remove_invalid_options will not
try to filter the search_options.So for admin user, the 'limit'
will appear in search_options, then obviously get no result.

fixed bug #1205956

Change-Id: Ib1a66c9d104ac52d6eae18be7f06d02985d4c2fd