CA-120846: fix get_mac for bonded interfaces

When an interface is bonded by the Linux bonding driver, the driver may
change the MAC address of the interface to that of the bond. The only
place where you can find the "real" MAC address of a bond slave seems to
be /proc/net/bonding.

Imported from xcp-networkd.git e976b03bbf5262cfb3c0e18382700396dcc54ef6

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Imported-by: John Else <john.else@citrix.com>
Merge pull request #1690 from johnelse/ca-120846-backport

CA-120846: fix get_mac for bonded interfaces