pagemap-cache: add const-qualifier to pmc's vma

We need to perform dirty page tracking when dumping shmem but there
we have only const vmas so we need pmc to work with them. Also pmc concept
implies that it won't change its vmas so it would be natural to declared
them as const.

Signed-off-by: Fyodor Bocharov <fbocharov@yandex.ru>
Signed-off-by: Eugene Batalov <eabatalov89@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>