client: hold reference for newly updated snaprealm

client: hold reference for newly updated snaprealm

Client::update_snap_trace() may create new snaprealm, then update
them. When Client::update_snap_trace() return, the newly created
snaprealm get freed immediately. This is wrong because callers of
Client::update_snap_trace() expects Client::get_snap_realm() return
the updated snaprealm.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>