Description of problem:
It should report an error while add an invalid pci controller to the guest

Version-Release number of selected component (if applicable):
libvirt-1.1-0.el7.x86_64
qemu-kvm-1.5.1-2.el7.x86_64
kernel-3.10.0-0.rc7.64.el7.x86_64
How reproducible:
100%

Steps
1.Prepare a normal guest
virsh list
 Id    Name                           State
----------------------------------------------------
 -     rhel72                     shutoff

2.Edit the guest's xml and add the following content to the guest
--
 <controller type='pci' index='-1' model='pci-bridge'/>
--
# virsh dumpxml rhel72|grep pci-bridge
    <controller type='pci' index='-1' model='pci-bridge'>

3.Start the guest,the guest will fail to start
# virsh start rhel72
error: Failed to start domain rhel72
error: Unable to read from monitor: Connection reset by peer
check the qemu's log
LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin QEMU_AUDIO_DRV=spice /usr/libexec/qemu-kvm -name rhel72 -S -machine pc-i440fx-rhel7.0.0,accel=kvm,usb=off -m 4001 -realtime mlock=off -smp 2,sockets=2,cores=1,threads=1 -uuid 403f2468-65cb-6be8-3735-4f654d3d5e4c -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/rhel72.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -global PIIX4_PM.disable_s3=0 -global PIIX4_PM.disable_s4=0 -device pci-bridge,chassis_nr=-1,id=pci.-1,bus=pci.0,addr=0x9 -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -device piix3-usb-uhci,id=usb1,bus=pci.0,addr=0x7 -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x5 -drive file=/mnt/zhwang/rhel7raw.img,if=none,id=drive-virtio-disk0,format=raw,cache=none -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x8,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -netdev tap,fd=23,id=hostnet0 -device rtl8139,netdev=hostnet0,id=net0,mac=52:54:004a:03,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -chardev socket,id=charchannel0,path=/var/lib/libvirt/qemu/rhel72.agent,server,nowait -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=org.qemu.guest_agent.0 -chardev spicevmc,id=charchannel1,name=vdagent -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,id=channel1,name=com.redhat.spice.0 -spice port=5900,addr=0.0.0.0,disable-ticketing,seamless-migration=on -k en-us -device qxl-vga,id=video0,ram_size=67108864,vram_size=67108864,bus=pci.0,addr=0x2 -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6
char device redirected to /dev/pts/1 (label charserial0)
qemu-kvm: -device pci-bridge,chassis_nr=-1,id=pci.-1,bus=pci.0,addr=0x9: Parameter 'chassis_nr' expects uint8_t
2013-07-04 06:57:45.873+0000: shutting down

4.Delete the upper invalid controller, then, edit the guest's xml add the following content to the guest
<controller type='pci' index='-1' model='pci-root'/>
<controller type='pci' index='-1' model='pci-root'/>
<controller type='pci' index='-1' model='pci-root'/>

# virsh dumpxml rhel72 |grep pci-root
    <controller type='pci' index='-1' model='pci-root'/>
    <controller type='pci' index='-1' model='pci-root'/>
    <controller type='pci' index='-1' model='pci-root'/>
    <controller type='pci' index='0' model='pci-root'/>            --this was valid one

5.Start the guest,the invalid pci controller didn't effect the guest start,and we can still see them while guest start completely.
# virsh start rhel72
Domain rhel72 started

# virsh dumpxml rhel72 |grep pci-root
    <controller type='pci' index='-1' model='pci-root'>
    <controller type='pci' index='-1' model='pci-root'>
    <controller type='pci' index='-1' model='pci-root'>
    <controller type='pci' index='0' model='pci-root'>

Actual results:
libvirt didn't check the the invalid pci controllers

Expected results:
It should report an error while add an invalid pci controller to the guest