Remove vsys_available method.

This fix removes vsys_available and relies on VsysFrontend checks for
the presence of vsys target FIFOs. While earlier versions of
read_vsys_data optionally ran a local command rather than using vsys,
this behavior has been removed, and vsys_available is no longer
necessary.
Clarify error message to reference missing "field".
Clarify VsysFrontend class docstring.
Clarify exception message if FIFOs are not found.
Combine logic in _setup_writer & _setup_reader.

Due to common code in these two methods, this patch replaces both with a
more generic _open_fifo method.
Update docstrings in VsysFrontend methods.

This patch clarifies the docstrings on VsysFrontend private methods.
Also, the new line appending to sent messages is moved from _send to
sendrecv for simplicity.
Fix quotes around message in VsysFrontend._recv
Clarify docstring for slicename_to_hostname.
Move logging for missing vserver names.

When a found xid is not listed in the known set of vserver names, a
current list is requested. However, if it is still not found, this is an
error because an xid is active in the system that has no name. So, this
patch logs this situation as an error.
Remove out of place comment.
Clarify parse_config docstring and inner variables.
Print warning for bad ExcludeSlice directives.

Only one slice name should be given to each ExcludeSlice directive. This
patch includes clearer conditions and logs a warning if the directive
includes the wrong number of parameters.
Fix typo for 'set up'.
Correct example for using custom request/response.
Correct docstring typo for FakeVsysBackend constructor.
Clarify vsys backend comment.
Fix formating for test parameters.

After changing some test names to a shorter form, parameters were
not moved to adjust for the shorter test names. This patch fixes the
test & parameter format.
Correct test name to reflect test content.
Complete variable name change (missed earlier).
Replace custom log method with logging.debug.