tests/functional/virt: drop testInitramfsReadable

testInitramfsReadable fails when the test framework creates its own
initramfs image, instead of using the host's. Such condition is
unfavorable, since it wastes time and may cause failures to boot the
guest, but it is not a failure per se.

Change-Id: I537b4d56ec6ca4a849c79590c7d934be6e70459f
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21854
Reviewed-by: Martin Polednik <mpoledni@redhat.com>