xen/pciback: simplify pcistub device handling

xen/pciback: simplify pcistub device handling

The Xen pciback driver maintains a list of all its seized devices.
There are two functions searching the list for a specific device with
basically the same semantics just returning different structures in
case of a match.

Split out the search function.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
OraBug: 25497392

(cherry picked from commit 1af916b701db1a9905e559e742f45818eb233d12)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>