qga: add ssh-get-authorized-keys

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
*fix-up merge conflicts due to qga-ssh-test being disabled in earlier
 patch due to G_TEST_OPTION_ISOLATE_DIRS triggering build-oss-fuzz
 leak detector.
*fix up style and disallowed g_assert* usage reported by checkpatch
Signed-off-by: Michael Roth <michael.roth@amd.com>