libxc: don't populate same pfn more than once in populate_pfns

libxc: don't populate same pfn more than once in populate_pfns

The original implementation of populate_pfns didn't consider the same
pfn can be present multiple times in the array. The mechanism to prevent
populating the same pfn multiple times only worked if the recurring pfn
appeared in different batches.

This bug is discovered by Linux 4.1 32 bit kernel save / restore test,
which has several ptes pointing to same pfn, which results in an array
containing recurring pfn.  When libxc called x86_pv_localise_page, the
original implementation would populate the same pfn more than once.

The fix is to set bit in populated bitmap as we generate list of pfns to
be populated.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>