backend: Add function to check whether file is in queue dir
backend: Add function to check whether file is in queue dir

Another function will need to check whether its parameters
are job queue files.

Reviewed-by: iustinp


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