qemuimage-testlib: Add delay to work around races in qemu startup

Qemu changes pid when starting up. On a loaded machine, this can result
in the incorrect pid being returned. Since qemu will take a few seconds to
boot anyway, we might as well delay a short while and allow things to settle
which should fix various race issues being seen on the autobuilder.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>