x86: add new Xen cpuid node for max address width info

x86: add new Xen cpuid node for max address width info

On very large hosts a pv-guest needs to know whether it will have to
handle frame numbers larger than 32 bits in order to select the
appropriate grant interface version.

Add a new Xen specific CPUID node to contain the maximum machine address
width similar to the x86 CPUID node 0x80000008 containing the maximum
physical address width. The maximum frame width needs to take memory
hotplug into account.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>