common/cmdparse: use string_view for the key

common/cmdparse: use string_view for the key

for better usability and performance. as the main use case of
cmd_getval() and cmd_putval() only uses a literal string for the key.
it's a waste to build a std::string out of it and throw it away after
looking the cmdmap with it.

Signed-off-by: Kefu Chai <kchai@redhat.com>