The migration code generally is built target-independent, however
there are a few places where knowing the target page size would
avoid artificially moving stuff into migration/ram.c.

Provide 'qemu_target_page_bits()' that returns TARGET_PAGE_BITS
to other bits of code so that they can stay target-independent.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NAmit Shah <amit.shah@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>