Add a new minimal OEQA test case to run the Xen Test Framework
test runner in the standard testimage step. Tested with qemux86-64
and designed for compatibility with Arm when XTF supports it.

To enable, append to local.conf:
    INHERIT += "testimage"
    QEMU_USE_SLIRP = "1"
    TEST_SERVER_IP = "127.0.0.1"

To run: bitbake -c testimage xtf-image

For inspection while it runs, at another shell prompt:
 * Observe the image booting:
 tail -f ${TMPDIR}/work/qemux86_64-*/xtf-image/*/testimage/qemu_boot_log.*
 * Observe the tests running once boot has completed:
 tail -f ${TMPDIR}/work/qemux86_64-*/xtf-image/*/temp/log.do_testimage

The XTF test sequence by default is a single XTF test case with minimal
hardware dependency to ensure that Xen is running, the Xen toolstack is
functional and XTF works. Additional XTF test cases for an image can be
configured via variables that are documented in the OEQA test case:
   * XTF_TEST_CASES_POPULATE
   * XTF_TEST_CASES_SKIP
   * XTF_TEST_CASES_REQUIRE

Since testimage requires a functioning network to perform the tests on
the image and the qemu MACHINES do not have networking enabled
this commit provides a new qemuboot-testimage-network bbclass to add an
image postprocess command to enable a functional eth0 for qemu MACHINES.

Signed-off-by: Christopher Clark <christopher.clark@starlab.io>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>