Made format similar to other plugins

plugin_allocate and plugin_preboot descriptions are implemented as
<variablelist></variablelist> elements.
Made plugin_disallocate format similar.
Added element tags for some attributes
Changed/added <command> element to commands
Proper name is Virtual Machine Manager

virt-manager is not Copyright by RedHat and full name is Virtual
Machine Manager.
Fixed names to match recent code changes
Minor spelling, grammar and caps fixes