qemu-doc: update deprecation section to use -nic and -netdev hubport

qemu-doc: update deprecation section to use -nic and -netdev hubport

The deprecated SLIRP options -tftp, -bootp, -redir, -smb provide
sample replacements that use "-net nic".  Suggest "-nic" instead,
since we finally have a path towards getting rid of "-net".

For "-net vlan" the replacement involves hubport network devices,
so mention that too.

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