xenconsoled: use array index to keep track of pollfd

xenconsoled: use array index to keep track of pollfd

If we use pointers to reference elements inside array, it is possible that we
get wild pointer after realloc(3) copies array and returns a new pointer.

Keep track of element indexes inside the array can solve this problem.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Marcus Granado <marcus.granado@citrix.com>
Tested-by: Marcus Granado <marcus.granado@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>