Fix possible invalid read in adminClientGetInfo

Fix possible invalid read in adminClientGetInfo

virNetServerClientGetInfo returns the client's remote address
as a string, which is a part of the client object.

Use VIR_STRDUP to make a copy which can be freely accessed
even after the virNetServerClient object is unlocked.

To reproduce, put a sleep between virObjectUnlock in
virNetServerClientGetInfo and virTypedParamsAddString in
adminClientGetInfo, then close the queried connection during
that sleep.