Update paths in man page to reflect libvirtd code

Update paths in man page to reflect libvirtd code

Updated the paths in the man page to reflect what the code in libvirtd
does. In addition broke out the FILES section into two subsections for
files used when run as root and files used when run as non-root.
Provided information about the defaults that libvirtd uses when running
as non-root and when XDG_CONFIG_HOME and XDG_RUNTIME_DIR are not set in
the environment.