Drop libguestfs-gobject-doc because gtk-doc is no longer provided upstream.
Add new man page guestfs-gobject(3) to libguestfs-gobject-devel.
Cherry picked from commit 768b0905ada0aa4db8a9b7178dcea0c020b8881a
and commit c7aa6a9556a64ad17ac207807f43775d0c466345.