Fix _RemoveDisk for file based instances
Fix _RemoveDisk for file based instances

During 621b7678 two typos were introduced which prevent file based
instances removal to work correctly. Fixing both of them to what they
were meant to be.

Signed-off-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

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