xm block-create doesn't work. It seems like this command hasn't even

xm block-create doesn't work.  It seems like this command hasn't even
been tested (perhaps since the un-Twisting?).  This particular problem
was that one function was being called with self instead of the right
argument and another function's return value was being used when it
didn't actually return anything.

This patch also improves the error handling a bit by making sure we
don't thrown an exception on a log statement with a None value.  In
general, one should always use the % formatter instead of concatination
for strings in Python (even though this is not what this patch does).

Signed-off-by: Anthony Liguori