x86: map frame table sparsely

x86: map frame table sparsely

Avoid backing frame table holes with memory, when those holes are
large enough to cover an exact multiple of large pages. This is based
on the introduction of a bit map, where each bit represents one such
range, thus allowing mfn_valid() checks to easily filter out those
MFNs that now shouldn't be used to index the frame table.

This allows for saving a couple of 2M pages even on "normal" systems.

Signed-off-by: Jan Beulich <jbeulich@novell.com>