conf: Add support for specifying CPU max physical address size

This patch introduces the

    <maxphysaddr mode='passthrough'/>
    <maxphysaddr mode='emulate' bits='42'/>

sub element of /domain/cpu, which allows specifying the guest virtual CPU
address size. This can be useful if the guest needs to have a large amount
of memory.

If mode='passthrough', the virtual CPU will have the same number of address
bits as the host. If mode='emulate', the mandatory bits attribute specifies
the number of address bits.

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>