Hi,
Under 5.1.0-BETA3, using OpenIndiana 5.11 and FreeBSD 10.3 guests the cursor is either a full blob (OpenIndiana using Gnome2 and the 5.1.0 guest additions) or partial blob with an arrow within it (FreeBSD using Xorg and and a lightweight Motif window manager using the emulators/virtualbox-additions FreeBSD package). There is no problem using CentOS7. I haven't tested CentOS5, CentOS6, or the latest Fedora yet.