admin: include: Introduce some client's identity related typed params macros

admin: include: Introduce some client's identity related typed params macros

This patch could easily be squashed with the virAdmClientGetInfo method
introduced later one, but the idea was to split the logic to as many preferably
independent patches as possible. As the subject hints, this patch defines
some public typed params-related macros used within virAdmClientGetInfo
method. The thing is, there's one identity attribute missing in the set --
SELinux context, which libvirt internally supports in virIdentity, but it
doesn't seem to do much (or anything at all), so there's some room to extend
the set in the future.

Signed-off-by: Erik Skultety <eskultet@redhat.com>