Add a new OpVerifyDisks opcode
Add a new OpVerifyDisks opcode

This patch adds the definition of a new opcode that will be used to
compute the list of instances with not-online disks.

Reviewed-by: imsnah


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