Review checks for Error string when checking that bad arguments are handled

Review checks for Error string when checking that bad arguments are handled
correctly.  Many checks were looking using output.find("Error") > 1, which is
incorrect, as we do not guarantee that the word "Error" will be first in the
output (never mind the fact that strings are indexed from 0).

All these checks have been changed to compare against -1.  In particular, this
should fix the failure of create_noparm_neg and create_badparm_neg on a machine
with the /etc/xen/xmdefconfig file in place, as xm create issues the
Using config file "/etc/xen/xmdefconfig" diagnostic before reporting the error.

Signed-off-by: Ewan Mellor <ewan@xensource.com>