Fix failure to create q35 machine This is a regression introduced by c0907c9e6417c. How to reproduce: $ qemu-system-x86_64 -nodefaults -vnc :0 -M q35 qemu-system-x86_64: Unknown device 'q35-pcihost' for default sysbus Aborted (core dumped) Tested-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Hu Tao <hutao@cn.fujitsu.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>