Check vxlan enablement via modinfo

Check vxlan enablement via modinfo

On some linux distribution, for RHEL 6.5 as an example, vxlan is
enabled but the kernel version is still 2.6. And some linux kernel
version is higher than 3.8 or even 3.11, its vxlan mod may be
disabled. Under both situation, vxlan enablement checking via linux
kernel version may not be correct.

In this patch, we check vxlan enablement via modinfo: if vxlan module
exists and functional test pass, vxlan is enabled.

Closes-Bug: #1267682

Change-Id: Id52c04b4739d2d11fe52d4b1631cb4f39e6b577f