qemu: report an error if usb keyboards are unsupported
qemu: report an error if usb keyboards are unsupported

Be nicer to the user and report a proper error instead of:
An error occurred, but the cause is unknown

https://bugzilla.redhat.com/show_bug.cgi?id=1460086

libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,datto/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,nertpinx/libvirt,andreabolognani/libvirt,nertpinx/libvirt,andreabolognani/libvirt,crobinso/libvirt,nertpinx/libvirt,datto/libvirt,datto/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,eskultety/libvirt,eskultety/libvirt,eskultety/libvirt,jardasgit/libvirt,jfehlig/libvirt,jardasgit/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,olafhering/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,crobinso/libvirt,nertpinx/libvirt,eskultety/libvirt,fabianfreyer/libvirt,olafhering/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,datto/libvirt,libvirt/libvirt,eskultety/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,datto/libvirt,zippy2/libvirt,jardasgit/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt