Fix swupdate listing

In ubuntu 13.10, package does not have architecture method,
using pkg.architecture() raises error.
workaround by using _pkg.architecture.

dir(pkg) output:
['_Package__set_candidate', '__class__', '__delattr__',
'__dict__', '__doc__', '__format__', '__getattribute__',
'__hash__', '__init__', '__lt__', '__module__', '__new__',
'__reduce__', '__reduce_ex__', '__repr__', '__setattr__',
'__sizeof__', '__str__', '__subclasshook__', '__weakref__',
'_changelog', '_pcache', '_pkg', 'candidate', 'commit', 'essential',
'fullname', 'get_changelog', 'id', 'installed', 'installed_files',
'is_auto_installed', 'is_auto_removable', 'is_inst_broken',
'is_installed', 'is_now_broken', 'is_upgradable', 'mark_auto',
'mark_delete', 'mark_install', 'mark_keep', 'mark_upgrade', 'marked_delete',
'marked_downgrade', 'marked_install', 'marked_keep', 'marked_reinstall',
'marked_upgrade', 'name', 'section', 'shortname', 'versions']

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>