Fix failure to create q35 machine

This is a regression introduced by c0907c9e64. 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>

default-configs/i386-softmmu.mak

@@ -42,6 +42,6 @@ CONFIG_ISA_TESTDEV=y
 CONFIG_VMPORT=y
 CONFIG_SGA=y
 CONFIG_LPC_ICH9=y
-CONFIG_Q35=y
+CONFIG_PCI_Q35=y
 CONFIG_APIC=y
 CONFIG_IOAPIC=y

default-configs/x86_64-softmmu.mak

@@ -42,6 +42,6 @@ CONFIG_ISA_TESTDEV=y
 CONFIG_VMPORT=y
 CONFIG_SGA=y
 CONFIG_LPC_ICH9=y
-CONFIG_Q35=y
+CONFIG_PCI_Q35=y
 CONFIG_APIC=y
 CONFIG_IOAPIC=y