tests: Add vm test lib

This is the common code to implement a "VM test" to

  1) Download and initialize a pre-defined VM that has necessary
  dependencies to build QEMU and SSH access.

  2) Archive $SRC_PATH to a .tar file.

  3) Boot the VM, and pass the source tar file to the guest.

  4) SSH into the VM, untar the source tarball, build from the source.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
