netinfo/sampling: split speed into nic and bond specific methods

Speed is calculated very differently for nics and bonds. This patch
splits the logic into two different methods and makes netinfo and
sampling use them.

Change-Id: If8d02ce7dec2cafec99478f3b5f089f1fce76d7e
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21542
Reviewed-by: Dan Kenigsberg <danken@redhat.com>