qapi-schema: Rename SocketAddressFlat's variant tcp to inet

qapi-schema: Rename SocketAddressFlat's variant tcp to inet

QAPI type SocketAddressFlat differs from SocketAddress pointlessly:
the discriminator value for variant InetSocketAddress is 'tcp' instead
of 'inet'.  Rename.

The type is so far only used by the Gluster block drivers.  Take care
to keep 'tcp' working in things like -drive's file.server.0.type=tcp.
The "gluster+tcp" URI scheme in pseudo-filenames stays the same.
blockdev-add changes, but it has changed incompatibly since 2.8
already.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>