The target_dumper property is only set by the QemuTarget subclass, so
assign a default value and check it isn't None before calling it.
(From OE-Core rev: add36cf85380bf7dc5339a9d65f951923317440b)
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>