Fix some errors in instance modify --disk remove
Fix some errors in instance modify --disk remove

The RpcResult introduction still left some bugs (after multiple patches):
  - we don't correctly check the result type
  - rename a variable to prevent a conflict

Reviewed-by: imsnah

ganeti/ganeti,dimara/ganeti,mbakke/ganeti,apyrgio/snf-ganeti,grnet/snf-ganeti,andir/ganeti,mbakke/ganeti,leshchevds/ganeti,yiannist/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,bitemyapp/ganeti,andir/ganeti,onponomarev/ganeti,apyrgio/ganeti,yiannist/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,yiannist/ganeti,andir/ganeti,apyrgio/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,leshchevds/ganeti,dimara/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti