msi: add Xen MSI implementation

This patch adds support for MSI interrupts when running on Xen. Apart
from adding the Xen related code needed in order to register MSI
interrupts this patch also makes the msi_init function a hook in
init_ops, so different MSI implementations can have different
initialization functions.

Sponsored by: Citrix Systems R&D

xen/interface/physdev.h:
 - Add the MAP_PIRQ_TYPE_MULTI_MSI to map multi-vector MSI to the Xen
   public interface.

x86/include/init.h:
 - Add a hook for setting custom msi_init methods.

amd64/amd64/machdep.c:
i386/i386/machdep.c:
 - Set the default msi_init hook to point to the native MSI
   initialization method.

x86/xen/pv.c:
 - Set the Xen MSI init hook when running as a Xen guest.

x86/x86/local_apic.c:
 - Call the msi_init hook instead of directly calling msi_init.

xen/xen_intr.h:
x86/xen/xen_intr.c:
 - Introduce support for registering/releasing MSI interrupts with
   Xen.
 - The MSI interrupts will use the same PIC as the IO APIC interrupts.

xen/xen_msi.h:
x86/xen/xen_msi.c:
 - Introduce a Xen MSI implementation.

x86/xen/xen_nexus.c:
 - Overwrite the default MSI hooks in the Xen Nexus to use the Xen MSI
   implementation.

x86/xen/xen_pci.c:
 - Introduce a Xen specific PCI bus that inherits from the ACPI PCI
   bus and overwrites the native MSI methods.
 - This is needed because when running under Xen the MSI messages used
   to configure MSI interrupts on PCI devices are written by Xen
   itself.

dev/acpica/acpi_pci.c:
 - Lower the quality of the ACPI PCI bus so the newly introduced Xen
   PCI bus can take over when needed.

conf/files.i386:
conf/files.amd64:
 - Add the newly created files to the build process.
xen: add the Xen implementation of pci_child_added method

Add the Xen specific implementation of pci_child_added to the Xen PCI
bus. This is needed so FreeBSD can register the devices it finds with
the hypervisor.

Sponsored by: Citrix Systems R&D

x86/xen/xen_pci.c:
 - Add the Xen pci_child_added method.
xen: make xen balloon a driver that depends on xenstore

This is done so we can prevent the Xen Balloon driver from attaching
before xenstore is setup.

Sponsored by: Citrix Systems R&D

dev/xen/balloon/balloon.c:
 - Make xen balloon a driver that depends on xenstore.