libdfu: Only enumerate the DfuContext when required
libdfu: Only enumerate the DfuContext when required

This speeds up afl fuzzing significantly.

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