Port -net none and -net nic to QemuOpts

Port -net none and -net nic to QemuOpts

We use a table of network types to look up the initialization function
and parameter descriptions in net_client_init().

For now, we use QemuOpts for the 'none' and 'nic' types. Subsequent
patches port the other types too and the special casing is removed.

We're not parsing the full -net option string here as the type has
been stripped from the string, so we do not use qemu_opts_parse()
'firstname' facility. This will also be rectified in subsequent
patches.

No functional changes are introduced by this patch.

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