Fix modification of instance memory
Fix modification of instance memory

... as found by the QA script - bug was introduced by me in commit 2117.

Reviwed-by: imsnah

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