handbook/virtualization: Add section on VBoxSVGA to make Xorg work

When trying to run Xorg on FreeBSD as a VirtualBox guest, it fails to
start the X server because of the wrong graphics driver. Instruct users
to set the VBoxSVGA graphics adapter before installing the guest
additions.

Also, remove the "First," from the beginning of a later sentence, as
there is no "Second", "Third", and so on and does not help much in this
context.

Event: BSDCan 2026
Reviewed by: ziaee@
Differential Revision: https://reviews.freebsd.org/D57693