Make qemux86-64 image using Yocto, convert it to Mender image and run acceptance tests

Acceptance criteria:

• mender-convert tests must do the following:
  • Build a vanilla qemux86-64 image from Yocto (most likely core-image-full-cmdline, like we do for meta-mender builds)
  • Convert that image to a Mender image
  • Run mender-image-tests tests on it

This covers x86.