docs: update docs for setting the QEMU BIOS path
commit ddf2dfa1f79af0405df5ca10583764a497c7a0db
provided a way to determine which bios files to use.
But we need to update related docs.
disccussed at:
http://www.redhat.com/archives/libvir-list/2012-June/msg01286.html
Signed-off-by: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>