test/spdkcli: test set_coalescing method

Add spdkcli command that will invoke set_coalescing method
  during vhost test.
This is the only usage of set_coalescing in our tests,
  so it would be nice to at least have an invocation of it.

Following commits will add a match-test that will check if
  set_coalescing worked as expected.

This patch is related to trello task:
https://trello.com/c/CHOOxcGj/151-spdkcli-extend-test-coverage



Change-Id: I15f54d92c4af2cf3f400743425e268b3645a5b3c
Signed-off-by: Vitaliy Mysak <vitaliy.mysak@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/462859


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Pawel Kaminski <pawelx.kaminski@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>