src/dpdk: Enable building of vhost-user in src/dpdk.

src/dpdk: Enable building of vhost-user in src/dpdk.

Allow vhost-user compilation by specifying make VHOST_USER=y
in src/Makefile. This commit also fixes a dpdk linking issue
by modifying config/common_linuxapp instead of passing args.

JIRA: VSPERF-56
Change-Id: I0437bf92b37f8f5a1939690c308cd54dfbe017f0
Signed-off-by: Dino Simeon Madarang <dino.simeonx.madarang@intel.com>
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Billy O Mahony <billy.o.mahony@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Eugene Snider <Eugene.Snider@huawei.com>
Reviewed-by: Gurpreet Singh <gurpreet.singh@spirent.com>
Reviewed-by: Tv Rao <tv.rao@freescale.com>