pygrub: correct typo in --args assignment

pygrub: correct typo in --args assignment

If pygrub was called with --args="some thing", then this string should
be append to the kernel command line.  But the last changeset
25941:795c493fe561 contained a typo, it assigns 'args' instead of 'arg'.

Rename the local variable which holds the string from the domain config
file to avoid further confusion.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>