ux: describe better how to reboot VM

Describe better how to reboot the VM during
setup.

Change-Id: Id63867e1f43dec3b2ef0c4a179f21ba1ac5876d1
Bug-Url: https://bugzilla.redhat.com/1001726
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>