pv-on-hvm: fixes for unmodified drivers build and modern Linux

pv-on-hvm: fixes for unmodified drivers build and modern Linux

- The adjustments to README and overrides.mk are generic.
- The removal of explicit linux/config.h inclusion should also not
  cause any issues.
- The introduction of irq_handler_t should eliminiate warnings on
  2.6.19+ kernels (I didn't check they're there, but since the
  request_irq prototype changed, I'm sure there's at least
  one. However, as a result changes to the Linux tree are expected to
  be required.
- The change setup_xen_features -> xen_setup_features follows the
  naming in mainline 2.6.23 but would apparently also require changes
  to the Linux tree.
- The changes SA_* -> IRQF_ and  pci_module_init ->
  pci_register_driver should also not cause issues.

Signed-off-by: Jan Beulich <jbeulich@novell.com>