Only return updatable devices from GetDevices()
Only return updatable devices from GetDevices()

Fixes some of https://github.com/hughsie/fwupd/pull/47

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