doc: Recommend '-enable-kvm' for installing in a VM as well.

* doc (Installing Guix in a VM): Add '-enable-kvm' to example.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -2430,13 +2430,14 @@ The resulting file will be much smaller than 50 GB (typically less than
 Boot the USB installation image in an VM:
 @example
-qemu-system-x86_64 -m 1024 -smp 1 \
+qemu-system-x86_64 -m 1024 -smp 1 -enable-kvm \
   -net user -net nic,model=virtio -boot menu=on \
   -drive file=guix-system-install-@value{VERSION}.@var{system}.iso \
   -drive file=guixsd.img
 @end example
-The ordering of the drives matters.
+The ordering of the drives matters.  @code{-enable-kvm} is optional, but
+significantly improves performance, @pxref{Running Guix in a VM}.
 In the VM console, quickly press the @kbd{F12} key to enter the boot
 menu.  Then press the @kbd{2} key and the @kbd{RET} key to validate your