vfio/type1: Change success value of vaddr_get_pfn()

vaddr_get_pfn() simply returns 0 on success.  Have it report the number
of pfns successfully gotten instead, whether from page pinning or
follow_fault_pfn(), which will be used later when batching pinning.

Change the last check in vfio_pin_pages_remote() for consistency with
the other two.

Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>