tests/9p: merge *walk*() functions

tests/9p: merge *walk*() functions

Introduce declarative function calls.

There are currently 4 different functions for sending a 9p 'Twalk'
request: v9fs_twalk(), do_walk(), do_walk_rqids() and
do_walk_expect_error(). They are all doing the same thing, just in a
slightly different way and with slightly different function arguments.

Merge those 4 functions into a single function by using a struct for
function call arguments and use designated initializers when calling
this function to turn usage into a declarative approach, which is
better readable and easier to maintain.

Also move private functions genfid(), split() and split_free() from
virtio-9p-test.c to virtio-9p-client.c.

Based-on: <E1odrya-0004Fv-97@lizzy.crudebyte.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <607969dbfbc63c1be008df9131133711b046e979.1664917004.git.qemu_oss@crudebyte.com>