xen/pciback: avoid multiple entries in slot list

The Xen pciback driver has a list of all pci devices it is ready to
seize. There is no check whether a to be added entry already exists.
While this might be no problem in the common case it might confuse
those which consume the list via sysfs.

Modify the handling of this list by not adding an entry which already
exists. As this will be needed later split out the list handling into
a separate function.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>