xend vs xl with pci=['<bdf'] wherein the '<bdf>' are not owned by pciback or pcistub will still launch.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

In Xend, if I had a pci entry in the guest config and the 
PCI device was not assigned to xen-pciback or pci-stub it
would refuse to launch the guest.

Not so with 'xl'. It will complain but still launch:

-bash-4.1# cd drivers/pciback/
-bash-4.1# ls
0000:01:00.0  0000:03:08.1  0000:03:0a.0  0000:03:0b.1       irq_handlers  new_slot    remove_id    uevent
0000:01:00.1  0000:03:09.0  0000:03:0a.1  bind               module        permissive  remove_slot  unbind
0000:03:08.0  0000:03:09.1  0000:03:0b.0  irq_handler_state  new_id        quirks      slots
-bash-4.1# echo "0000:03:0b.0" > unbind
-bash-4.1# echo "0000:03:0b.1" > unbind
-bash-4.1# xl create /mnt/lab/security/security.cfg  
Parsing config from /mnt/lab/security/security.cfg
libxl: error: libxl_pci.c:1055:libxl__device_pci_add: PCI device 0:3:b.0 is not assignable
libxl: error: libxl_pci.c:1055:libxl__device_pci_add: PCI device 0:3:b.1 is not assignable
-bash-4.1# xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0  2047     4     r-----      14.7
security                                     1  1023     1     -b----       8.0
-bash-4.1# 
-bash-4.1# cat /mnt/lab/security/security.cfg |grep -v \#
device_model_version="qemu-xen-traditional"
builder="hvm"
memory = 1024
name = "security"
vcpus=1
vif = [ 'mac=00:0F:4B:00:00:84,bridge=switch' ]
disk = [ 'phy:/dev/sda,xvda,w' ]
pci= ['0000:03:08.0', '000:03:08.1', '0000:03:09.0', '0000:03:09.1', '0000:03:0a.0', '0000:03:0a.1', '0000:03:0b.0', '0000:03:0b.1']
vnc=1
vnclisten='0.0.0.0'
vncunused=1
serial="pty"


And naturally when shutting/destroying the guest it will say:
-bash-4.1# xl destroy 1
libxl: error: libxl_pci.c:1265:do_pci_remove: xc_deassign_device failed: No such device
libxl: error: libxl_pci.c:1265:do_pci_remove: xc_deassign_device failed: No such device

(XEN) [2014-01-06 17:54:39] deassign 0000:03:0b.0 from dom1 failed (-19)
(XEN) [2014-01-06 17:54:39] deassign 0000:03:0b.1 from dom1 failed (-19)

because it tries to de-allocate them even though they were not
part of the guest.