PAPI stats: Use list as argument to ls

PAPI stats: Use list as argument to ls

In the VPP PAPI stats code, the "ls" argument has always [0]
been named "patterns", suggesting it expects a list of strings.
In the CSIT utility we have always [1] been using it wrong
(passing a single string), but it happened to work
as VPP code had a workaround [2] for that.
This workaround that got removed by accident in [3],
and will be reintroduced in [4].

This change fixes occasional failures in CSIT
(if merged sooner than [4]), mainly useful
in aviding such failures during bisection.

[0] https://gerrit.fd.io/r/c/vpp/+/14584/4/src/vpp-api/python/vpp_papi/vpp_stats.py#122
[1] https://gerrit.fd.io/r/c/csit/+/18419/13/resources/tools/papi/vpp_papi_provider.py#181
[2] https://gerrit.fd.io/r/c/vpp/+/14584/4/src/vpp-api/python/vpp_papi/vpp_stats.py#64
[3] https://gerrit.fd.io/r/c/vpp/+/31678
[4] https://gerrit.fd.io/r/c/vpp/+/31817

Change-Id: Ic0f9d906be064b386b67154ba738646c4362aa2b
Signed-off-by: Vratko Polak <vrpolak@cisco.com>