libxc: correct max_pfn calculation for saving domain

libxc: correct max_pfn calculation for saving domain

Commit 91e204d37f44913913776d0a89279721694f8b32 ("libxc: try to find
last used pfn when migrating") introduced a bug for the case of a
domain supporting the virtual mapped linear p2m list: the maximum pfn
of the domain calculated from the p2m memory allocation might be too
low.

Correct this.

Reported-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>