Added test_create.py, a test script for create.py. This contains a unit test

Added test_create.py, a test script for create.py.  This contains a unit test
for the command line parsing code in xm create.  To make it easier to test,
create.py has changed in structure around the main(argv) function, adding a new
parseCommandLine function.

Remove a large number of useless parameters, where the opts structure was being
passed all over the place, just to eventually log through it.  Instead two
methods -- err and warn -- have been broken out from the Opts class, and there
is now no need to pass the opts value everywhere.

Change the parsing inside make_config to remove lots of cut-and-pasted cruft.

Change the documentation to the vif command to indicate that the bridge can be
auto-detected.

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