Import selected headers from the xen/include/public directory of the
Xen repository at git://xenbits.xen.org/xen.git

The script ./include/xen/import.pl can be used to automatically import
any required headers and their dependencies (in a similar fashion to
./include/ipxe/efi/import.pl).  Trailing whitespace is stripped and an
appropriate FILE_LICENCE declaration is added to each header file.

Signed-off-by: Michael Brown <mcb30@ipxe.org>