xen-pciback: fix up cleanup path when alloc fails

xen-pciback: fix up cleanup path when alloc fails

When allocating a pciback device fails, clear the private
field. This could lead to an use-after free, however
the 'really_probe' takes care of setting
dev_set_drvdata(dev, NULL) in its failure path (which we would
exercise if the ->probe function failed), so we we
are OK. However lets be defensive as the code can change.

Going forward we should clean up the pci_set_drvdata(dev, NULL)
in the various code-base. That will be for another day.

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reported-by: Jonathan Creekmore <jonathan.creekmore@gmail.com>
Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit 584a561a6fee0d258f9ca644f58b73d9a41b8a46)
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>