It turned out (surprise surprise) that page server exchanges CR_FD_PAGEMAP
and CR_FD_SHMEM_PAGEMAP values in ps_iov_msg. The problem is that these
constants sit in enum and change their values from version to version %)

So here's the fixed version of the protocol including the backward compat
checks on all the values that could be met from older CRIUs (we're lucky
and they didn't intersect).
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>