xen/balloon: don't set P2M entry for auto translated guest

In commit cd9151e2

: xen/balloon: set a mapping for ballooned out pages
we have the ballooned out page's mapping set to a scratch page.

That commit also sets the P2M entry of ballooned out page to the scratch
page's MFN. This is necessary for PV guest but not for HVM guest. On the
other hand, setting the P2M entry would trigger BUG_ON in
__set_phys_to_machine.

The correct thing to do here is to avoid calling __set_phys_to_machine
for auto translated guest.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>