BACKPORT: libnvdimm: nd_region flush callback support

This patch adds functionality to perform flush from guest
to host over VIRTIO. We are registering a callback based
on 'nd_region' type. virtio_pmem driver requires this special
flush function. For rest of the region types we are registering
existing flush function. Report error returned by host fsync
failure to userspace.

Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
(cherry picked from commit c5d4355d10d414a96ca870b731756b89d068d57a)
[adelva: backport around some nvdimm refactors in >4.14]
Bug: 146400078
Bug: 148297388
Change-Id: Icf6ff5327b3c74455a4d53d2d37ac7fef7fbda85
Signed-off-by: Alistair Delva <adelva@google.com>