convert net_client_init() to OptsVisitor

The net_client_init() prototype is kept intact.

Based on "is_netdev", the QemuOpts-rooted QemuOpt-list is parsed as a
Netdev or a NetLegacy. The original meat of net_client_init() is moved to
and simplified in net_client_init1():

Fields not common between -net and -netdev are clearly separated. Getting
the name for the init functions is cleaner: Netdev::id is mandatory, and
all init functions handle a NULL NetLegacy::name. NetLegacy::vlan
explicitly depends on -net (see below).

Verifying the "type=" option for -netdev can be turned into a switch.

Format validation with qemu_opts_validate() can be removed because the
visitor covers it. Relatedly, the "net_client_types" array is reduced to
an array of init functions that can be directly indexed by opts->kind.
(Help text is available in the schema JSON.)

The outermost negation in the condition around qemu_find_vlan() was
flattened, because it expresses the dependent code's requirements more
clearly.

VLAN lookup is avoided if there's no init function to pass the VLAN to.

Whenever the value of type=... is needed, we substitute
NetClientOptionsKind_lookup[kind].

The individual init functions are not converted yet, thus the original
QemuOpts instance is passed transparently.

v1->v2:
- NetLegacy::name is optional. Tracked it through all init functions: they
  all handle a NULL name. Updated commit message accordingly.

v2->v3:
- NetLegacy::id is allowed and takes precedence over NetLegacy::name.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>