We need to do one more thing before we reboot to our new kernel. We have to recompile the VirtualBox guest module so that things like screen resizing will still work when we reboot to the new kernel.

First, make sure you have the virtualbox-guest-dkms package installed:

sudo pacman -S virtualbox-guest-dkms
If you have it installed already, pacman will ask you if you want to reinstall. Say no, but note the version number of the module.

Recompile the VirtualBox guest module by running:

sudo dkms remove  vboxguest/version-number -k kernel-version
sudo dkms install vboxguest/version-number -k kernel-version

Substitute your vboxguest version number and your kernel version string.

warning: virtualbox-guest-dkms-4.3.26-2 is up to date -- reinstalling
resolving dependencies...
looking for conflicting packages...

Packages (1) virtualbox-guest-dkms-4.3.26-2

Total Installed Size:  5.26 MiB
Net Upgrade Size:      0.00 MiB

:: Proceed with installation? [Y/n]

sudo dkms remove  vboxguest/version-number -k kernel-version
sudo dkms install vboxguest/version-number -k kernel-version