virdbus: Remove redundant error macro

virdbus: Remove redundant error macro

This is the only callsite.

We drop use of localerror.name here, because it's not actually useful
to us: rather than the parameter name which received an invalid value
(which was assumed), it's actually the the dbus errno equivalent.
Just use the error string.

Acked-by: Eric Blake <eblake@redhat.com>