net: validate that ids are well formed

net: validate that ids are well formed

When a network or network device is created from the command line or HMP,
QemuOpts ensures that the id passes the id_wellformed check.  However,
QMP skips this:

   $ qemu-system-x86_64 -qmp stdio -S -nic user,id=123/456
   qemu-system-x86_64: -nic user,id=123/456: Parameter id expects an identifier
   Identifiers consist of letters, digits, -, ., _, starting with a letter.

   $ qemu-system-x86_64 -qmp stdio -S
   {"execute":"qmp_capabilities"}
   {"return": {}}
   {"execute":"netdev_add", "arguments": {"type": "user", "id": "123/456"}}
   {"return": {}}

After:

   $ qemu-system-x86_64 -qmp stdio -S
   {"execute":"qmp_capabilities"}
   {"return": {}}
   {"execute":"netdev_add", "arguments": {"type": "user", "id": "123/456"}}
   {"error": {"class": "GenericError", "desc": "Parameter "id" expects an identifier"}}

Validity checks should be performed always at the bottom of the call chain,
because QMP skips all the steps above.  At the same time we know that every
call chain should go through either QMP or (for legacy) through QemuOpts.
Because the id for -net and -nic is automatically generated and not
well-formed by design, just add the check to QMP.

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>