Doc: Pacemaker Remote: Clarify pcs add-guest explanation

Clarify explanation of what pcs cluster node add-guest is doing.

Remove "mile-high" section completely.

• The info fits better into the "How pcs Configures the Guest" section.
• The need for virsh dumpxml is unclear... the XML should already exist after we created the VM.
• pcs cluster node add-guest doesn't install packages, configure the firewall, or create a VirtualDomain resource. We did all that in earlier steps.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>