qemu: Don't detach devices if passthrough doesn't work
qemu: Don't detach devices if passthrough doesn't work

https://bugzilla.redhat.com/show_bug.cgi?id=1046919

If none (KVM, VFIO) of the supported PCI passthrough methods is known to
work on a host, it's better to fail right away with a nice error message
rather than letting attachment fail with a more cryptic message such as

    Failed to bind PCI device '0000:07:05.0' to vfio-pci: No such device

Signed-off-by: Jiri Denemark <62bdf77dc47919a4d59a91822129d14633cfca81@redhat.com>

agx/libvirt,shugaoye/libvirt,elmarco/libvirt,rlaager/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,taget/libvirt,jardasgit/libvirt,libvirt/libvirt,rlaager/libvirt,eskultety/libvirt,VenkatDatta/libvirt,olafhering/libvirt,andreabolognani/libvirt,agx/libvirt,nertpinx/libvirt,shugaoye/libvirt,andreabolognani/libvirt,jardasgit/libvirt,elmarco/libvirt,taget/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,shugaoye/libvirt,olafhering/libvirt,taget/libvirt,fabianfreyer/libvirt,crobinso/libvirt,fabianfreyer/libvirt,crobinso/libvirt,zippy2/libvirt,eskultety/libvirt,nertpinx/libvirt,rlaager/libvirt,datto/libvirt,jfehlig/libvirt,eskultety/libvirt,nertpinx/libvirt,jfehlig/libvirt,datto/libvirt,datto/libvirt,VenkatDatta/libvirt,rlaager/libvirt,VenkatDatta/libvirt,datto/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,taget/libvirt,taget/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,libvirt/libvirt,zippy2/libvirt,agx/libvirt,zippy2/libvirt,elmarco/libvirt,eskultety/libvirt,rlaager/libvirt,jardasgit/libvirt,eskultety/libvirt,agx/libvirt,nertpinx/libvirt,crobinso/libvirt,andreabolognani/libvirt,shugaoye/libvirt,andreabolognani/libvirt,libvirt/libvirt,agx/libvirt,libvirt/libvirt,elmarco/libvirt,datto/libvirt,elmarco/libvirt,olafhering/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,crobinso/libvirt,jardasgit/libvirt