libxc: try to find last used pfn when migrating

libxc: try to find last used pfn when migrating

For migration the last used pfn of a guest is needed to size the
logdirty bitmap and as an upper bound of the page loop. Unfortunately
there are pv-kernels advertising a much higher maximum pfn as they
are really using in order to support memory hotplug. This will lead
to allocation of much more memory in Xen tools during migration as
really needed.

Try to find the last used guest pfn of a pv-domu by scanning the p2m
tree from the last entry towards it's start and search for an entry
not being invalid.

Normally the mid pages of the p2m tree containing all invalid entries
are being reused, so we can just scan the top page for identical
entries and skip them but the first one.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
[ ijc -- added errno = E2BIG to one error path ]
Acked-by: Ian Campbell <ian.campbell@citrix.com>