Final net cleanup after conversion to QemuOpts

Final net cleanup after conversion to QemuOpts

Now that net_client_init() has no users, kill it off and rename
net_client_init_from_opts().

There is no further need for the old code in net_client_parse() either.
We use qemu_opts_parse() 'firstname' facitity for that. Instead, move
the special handling of the 'vmchannel' type there.

Simplify the vl.c code into merely call net_client_parse() for each
-net command line option and then calling net_init_clients() later
to iterate over the options and create the clients.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>