yocto/xen-tools: Ensure domU is "dead" when shutdown

According to [1]:
shutdown
  -w, --wait
    Wait for the domain to complete shutdown before returning. If given once,
    the wait is for domain shutdown or domain death. If given multiple times,
    the wait is for domain death only.

For the Reference Stack Integration Tests we need to ensure that the domU is
completely "dead" when calling "/etc/init.d/domains stop" command.

[1] - https://xenbits.xen.org/docs/unstable/man/xl.1.html#DOMAIN-SUBCOMMANDS



Issue-Id: SCM-5314
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: I566ce2b40e20ee611ae94123873d26b033cf971e