xmlrpc: Change filter(...)[0] to get(...)

xmlrpc: Change filter(...)[0] to get(...)

filter() returns a list or [], and get() returns an object or raises an
exception.

Currently, we use filter()[0] inside a try/except model.DoesNotExist
block. This doesn't work: if filter returns [] we'll get an exception
about list indices, not a DoesNotExist exception.

Switch these over to get(), allowing them to use the existing exception
flow.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephenfinucane@hotmail.com>