Fix the libdfu self test failure on s390 and ppc64
Fix the libdfu self test failure on s390 and ppc64

We have to be careful converting the little endian keys and values to and from
a 'bare' uint8 buffer.

hughsie/fwupd,superm1/fwupd,vathpela/fwupd,superm1/fwupd,vathpela/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,superm1/fwupd,vathpela/fwupd,vathpela/fwupd,fwupd/fwupd