tests: run a test inside a QEMU VM

tests: run a test inside a QEMU VM

Use the script test/run.py to run a test named test_vm_tap
inside a QEMU VM. The run script builds out a virtual env,
launches a light weight QEMU VM, mounts host directories,
starts VPP inside the VM and runs the test. The test named
test_vm_tap, creates two tap v2 interfaces in separate Linux
namespaces and using iPerf, streams traffic between the VM
and VPP. All data files are stored in the directory named
/tmp/vpp-vm-tests. To clean up, use the make test-wipe
command.
Usage:
test/run.py --vm --debug --test test_vm_tap

Type: improvement

Change-Id: I4425dbef52acee1e5b8af5acaa169b89a2c0f171
Signed-off-by: Naveen Joy <najoy@cisco.com>