packet lib: packet_base: add a method which makes '_class_prefixes' easy to use
packet lib: packet_base: add a method which makes '_class_prefixes' easy to use

the variable '_class_prefixes' of stringify.StringifyMixin is used to help the method 'from_jsondict()' to create the class instance.
'_class_prefixes' requires a list of the class names.
some classes have another list of the class names already.
this patch adds a method which makes '_class_prefixes' by using the existing list.

before applying this patch:
(e.g. ryu.lib.packet.ipv6)

    # append the name to another list
    @ipv6.register_header_type(inet.IPPROTO_HOPOPTS)
    class hop_opts(opt_header):
    ...

    @ipv6.register_header_type(inet.IPPROTO_DSTOPTS)
    class dst_opts(opt_header):
    ....

    # append the name again
    ipv6._class_prefixes = ['hop_opts', 'dst_opts', ...]

after applying this patch:
(e.g. ryu.lib.packet.ipv6)

    # append the name to another list
    @ipv6.register_header_type(inet.IPPROTO_HOPOPTS)
    class hop_opts(opt_header):
    ...

    @ipv6.register_header_type(inet.IPPROTO_DSTOPTS)
    class dst_opts(opt_header):
    ....

    # create the new list from another list
    ipv6.set_classes(ipv6._IPV6_EXT_HEADER_TYPE)

Signed-off-by: Yuichi Ito <7a83edf5b53f0b21d4182a3da7518164ce03dc3a@gmail.com>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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