conf: rework lxc_config_parse_arch()

conf: rework lxc_config_parse_arch()

Fix architecture parsing. So far we couldn't really differ between "want
default architecture" and "failed to parse requested architecture"
because the -1 return value means both. Fix this by using the return
value only to indicate success or failure and return the parsed
personality in a return argument.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>