tests/9p: merge v9fs_tattach(), do_attach(), do_attach_rqid()

As with previous patches, unify those 3 functions into a single function
v9fs_tattach() by using a declarative function arguments approach.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <a6756b30bf2a1b25729c5bbabd1c9534a8f20d6f.1664917004.git.qemu_oss@crudebyte.com>