Xen support for x86/generic was added in 1d6879ee. This commit also
enables it for x86/64.
This was successfully tested with Xen 4.5, although the serial console
is broken in the same way as x86/generic (see FS#787)
Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>
6548b34c3f