mem: check existence of parent images before dumping pages (v2)

When we are doing pre-dump, we splice pages in pipes and only then open
images and dump pages. But when we are splicing pages, we need to know
about existence of parent images. This patch adds a new call to determin
existence of parent images.

In addition this patch fixes a following issue:
CID 83244 (#1 of 1): Uninitialized pointer read (UNINIT)
14. uninit_use: Using uninitialized value xfer.parent.

v2: initialize unused field of struct page_server_iov, because it sends
in network.

CID 83451 (#1 of 1): Uninitialized scalar variable (UNINIT)
2. uninit_use_in_call: Using uninitialized value pi. Field pi.nr_pages
is uninitialized when calling write.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
check: don't lead fd

CID 83243 (#1 of 1): Resource leak (RESOURCE_LEAK)
6. leaked_handle: Handle variable fd going out of scope leaks the handle.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
jenkins: reduce number of jobs for executing tests

32 is too many for our test server

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>