ia64: Move the macro definitions related to MSI to one header file.

For kvm's MSI support, it needs these macros defined in ia64_msi.c, and
to avoid duplicate them, move them to one header file and share with
kvm.

Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com>
Acked-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>