librbd: trade a map<> for a plain arrary

librbd: trade a map<> for a plain arrary

for two reasons

* to avoid the overhead of copying the loop variables.
* no need to keep a map<> if what we need is just an arrary
* no need to keep a string if what we need is but an array of char

also mark the array "const", as we never change it.

Signed-off-by: Kefu Chai <kchai@redhat.com>