cli: Make append_parameter_to_args accept a bt_value value

append_parameter_to_args is used to append a --params=KEY=VALUE argument
to a list of arguments.  It currently assumes that VALUE is a simple
string.  We will want to generalize this to support arrays (in the form
of a bt_value of type array).  This patch does a bit of refactoring in
preparation of that, making append_parameter_to_args accept a value of
type bt_value* instead of const char*.

append_string_parameter_to_args is introduced as a wrapper, for those
spots that still want to pass a simple string value.  It converts the
string to a temporary bt_value of type string.

No functional changes intended.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>