when creating macvtap passthrough to SR-IOV VF in Kernel >= 3.13 an
upper_macvtap symbolic link is created. For Kernel < 3.13 the only
way to know it is by parsing the ip link show output and look for
macvtap[0-9]+@<vf ifname>.
This patch used the ip link show command to detects
macvtap assigned vf so that detection of macvtap assigned vf
will work on all kernels
Closes-Bug: #1523083
Change-Id: Icbc8d6af5c00d1453095e04dd779210dc7244c7f