Add the function lxc_config_parse_arch that parses an architecture string
(x86, i686, x86_64, amd64) and returns the corresponding personality. This
is required for lxc-attach, which accepts architectures independently of
lxc.arch. The parsing of lxc.arch now also uses the same function to ensure
consistency.
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>