Make Xen VT-d PCI attach/detach work

Make Xen VT-d PCI attach/detach work

The Xen code for making HVM VT-d PCI passthrough attach and detach
wasn't working properly:

1)  In xenDaemonAttachDevice(), we were always trying to reconfigure
a PCI passthrough device, even the first time we added it.  This was
because the code in virDomainXMLDevID() was not checking xenstore for
the existence of the device, and always returning 0 (meaning that
the device already existed).

2)  In xenDaemonDetachDevice(), we were trying to use "device_destroy"
to detach a PCI device.  While you would think that is the right
method to call, it's actually wrong for PCI devices.  In particular,
in upstream Xen (and soon in RHEL-5 Xen), device_configure is actually
used to destroy a PCI device.

To fix the attach
problem I add a lookup into xenstore to see if the device we are
trying to attach already exists.  To fix the detach problem I change
it so that for PCI detach (only), we use device_configure with the
appropriate sxpr to do the detachment.

* src/xen/xend_internal.c: don't use device_destroy for PCI devices
  and fix the other issues.
* src/xen/xs_internal.c src/xen/xs_internal.h: add
  xenStoreDomainGetPCIID()