util: Set backing file name for LOOP_GET_STATUS64 queries.

This is an issue for LXC loop devices when you are trying to get loop
devices info using `ioctl`. Modern apps uses `/sys/dev/block` to grab
information about devices, but if you use the method mention you won't
be able to retrive the associated file with that loop device. See
example below from cryptsetup sources:

    static char *_ioctl_backing_file(const char *loop)
    {
        struct loop_info64 lo64 = {0};
        int loop_fd;

        loop_fd = open(loop, O_RDONLY);
        if (loop_fd < 0)
            return NULL;

        if (ioctl(loop_fd, LOOP_GET_STATUS64, &lo64) < 0) {
            close(loop_fd);
            return NULL;
        }

        lo64.lo_file_name[LO_NAME_SIZE-2] = '*';
        lo64.lo_file_name[LO_NAME_SIZE-1] = 0;

        close(loop_fd);
        return strdup((char*)lo64.lo_file_name);
    }

It will return an empty string because lo_file_name was not set.
Function `virFileLoopDeviceOpenSearch()` is using `ioctl` to query data,
but it is not checking `lo_file_name` field.
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>

src/util/virfile.c

@@ -781,6 +781,14 @@ int virFileLoopDeviceAssociate(const char *file,
     memset(&lo, 0, sizeof(lo));
     lo.lo_flags = LO_FLAGS_AUTOCLEAR;
 
+    /* Set backing file name for LOOP_GET_STATUS64 queries */
+    if (virStrncpy((char *) lo.lo_file_name, file,
+                   strlen(file), LO_NAME_SIZE) < 0) {
+        virReportSystemError(errno,
+                             _("Unable to set backing file %s"), file);
+        goto cleanup;
+    }
+
     if ((fsfd = open(file, O_RDWR)) < 0) {
         virReportSystemError(errno,
                              _("Unable to open %s"), file);