Position entries of net_client_types according to the corresponding
values of NET_CLIENT_TYPE_*. The array size is now defined by
NET_CLIENT_TYPE_MAX. This will allow to obtain entries based on type
value in later patches.

At this chance rename NET_CLIENT_TYPE_SLIRP to NET_CLIENT_TYPE_USER for
the sake of consistency.

CC: Markus Armbruster <armbru@redhat.com>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>