xen/pci: make bus notifier handler return sane values

Notifier functions are expected to return NOTIFY_* codes, not -E...
ones. In particular, since the respective hypercalls failing is not
fatal to the operation of the Dom0 kernel, it must be avoided to
return negative values here as those would make it appear as if
NOTIFY_STOP_MASK wa set, suppressing further notification calls to
other interested parties (which is also why we don't want to use
notifier_from_errno() here).

While at it, also notify the user of a failed hypercall.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
[v1: Added dev_err and the disable MSI/MSI-X call]
[v2: Removed the disable MSI/MSI-X call]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>