handbook/virtualization: Fix freebsd-update syntax

Split freebsd-update fetch and install into two subcommands. If running
an EoL FreeBSD, running the two in one command can fail to perform the
install. This matches what was done elsewhere in the tree.

PR: 293517
Reported by: Graham Perrin
Signed-off-by: Kit Dallege <xaum.io@gmail.com>
Closes: https://github.com/freebsd/freebsd-doc/pull/632