Fix error message in attachInstanceDiskChecks
Fix error message in attachInstanceDiskChecks

Name the instance where disks are already attached to, which is not
necessarily the instance where we want to attach a disk to.
This fixes issue 1151.

Signed-off-by: Lisa Velden <f95ae155456787aef400a49db463a62bbef84ead@google.com>
Reviewed-by: Oleg Ponomarev <c100474c6090841e9d55cde74d38d81efd269b8d@google.com>

onponomarev/ganeti,mbakke/ganeti,yiannist/ganeti,yiannist/ganeti,leshchevds/ganeti,ganeti/ganeti,andir/ganeti,leshchevds/ganeti,ganeti/ganeti,andir/ganeti,leshchevds/ganeti,andir/ganeti,mbakke/ganeti,onponomarev/ganeti,ganeti/ganeti,yiannist/ganeti,mbakke/ganeti