x86/shadow: don't enable shadow mode with too small a shadow allocation (part 2)

x86/shadow: don't enable shadow mode with too small a shadow allocation (part 2)

Commit 2634b997af ("x86/shadow: don't enable shadow mode with too small
a shadow allocation") was incomplete: The adjustment done there to
shadow_enable() is also needed in shadow_one_bit_enable(). The (new)
problem report was (apparently) a failed PV guest migration followed by
another migration attempt for that same guest. Disabling log-dirty mode
after the first one had left a couple of shadow pages allocated (perhaps
something that also wants fixing), and hence the second enabling of
log-dirty mode wouldn't have allocated anything further.

Reported-by: James Wang <jnwang@suse.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>