tests: Fix testBondHwAddress for fedora kernel

In relatively new kernel, the hwaddr of bonding will be set to a
random address when the last slave is released. You can find it in:
https://github.com/torvalds/linux/blob/master/drivers/net/bonding/bond_main.c#L1824

So to make the testBondHwAddress pass on fedora, we need backup the
hwaddr before destroy it.

Change-Id: Ieb4158e678b6e5bfaa7ac5f7e12834cbeeab9572
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/19590
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>