Xen PCI passthrough: fix passthrough failure when no interrupt pin

Commit 5a11d0f7 mistakenly converted a log message into an error
condition when no pin interrupt is found for the pci device being
passed through. Revert that part of the commit.

Signed-off-by: Bruce Rogers <brogers@suse.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
(cherry picked from commit 0968c91c)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>