nbd: Tidy up blockdev-add interface

SocketAddress is a simple union, and simple unions are awkward: they
have their variant members wrapped in a "data" object on the wire, and
require additional indirections in C.  I intend to limit its use to
existing external interfaces, and convert all internal interfaces to
SocketAddressFlat.

BlockdevOptionsNbd is an external interface using SocketAddress.  We
already use SocketAddressFlat elsewhere in blockdev-add.  Replace it
by SocketAddressFlat while we can (it's new in 2.9) for simplicity and
consistency.  For example,

    { "execute": "blockdev-add",
      "arguments": { "node-name": "foo", "driver": "nbd",
                     "server": { "type": "inet",
		                 "data": { "host": "localhost",
				           "port": "12345" } } } }

becomes

    { "execute": "blockdev-add",
      "arguments": { "node-name": "foo", "driver": "nbd",
                     "server": { "type": "inet",
		                 "host": "localhost", "port": "12345" } } }

Since the internal interfaces still take SocketAddress, this requires
conversion function socket_address_crumple().  It'll go away when I
update the interfaces.

Unfortunately, SocketAddress is also visible in -drive since 2.8:

    -drive if=none,driver=nbd,server.type=inet,server.data.host=127.0.0.1,server.data.port=12345

Nobody should be using it, as it's fairly new and has never been
documented, so adding still more compatibility gunk to keep it working
isn't worth the trouble.  You now have to use

    -drive if=none,driver=nbd,server.type=inet,server.host=127.0.0.1,server.port=12345

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1490895797-29094-9-git-send-email-armbru@redhat.com

[mreitz: Change iotest 147 accordingly]

Because of this interface change, iotest 147 has to be adapted.
Unfortunately, we cannot just flatten all of the addresses because
nbd-server-start still takes a plain SocketAddress. Therefore, we need
both and this is most easily achieved by writing the SocketAddress into
the code and flattening it where necessary.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170330221243.17333-1-mreitz@redhat.com

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
