vma: Unify private VMAs testing

We have two helpers for VMA type testing: privately_dump_vma() and vma_priv(). They
work with different types but basically do the same: check if we should dump VMA into
the image and restore it back then.

Lets unify they both into common vma_entry_is_private() helper and vma_area_is_private()
for working with vma_area type.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>