page-xfer: bug, don't open page-read for shmem

it was error if come to that point from shmem.c
388L: err = open_page_xfer(&xfer, CR_FD_SHMEM_PAGEMAP, si->shmid);

Signed-off-by: Tikhomirov Pavel <snorcht@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>