These are now in unstable and need to be tested. You cannot install these using
mhwd - instead you'll need to install kernel and all necessary extramodules manually.
After that, though, you should pick up new kernels automatically, and there will be a good test of this shortly with the release of kernel 5.2.
All this metapackage does is install the related kernel packages - the end result is the same as installing the kernel packages:
$ sudo pacman -S linux-latest linux-latest-virtualbox-host-modules linux-latest-zfs resolving dependencies... looking for conflicting packages... Packages (6) linux51-5.1.15-1 linux51-virtualbox-host-modules-6.0.8-12 linux51-zfs-0.8.1-5 linux-latest-5.1-1 linux-latest-virtualbox-host-modules-5.1-1 linux-latest-zfs-5.1-1 Total Installed Size: 131.50 MiB :: Proceed with installation? [Y/n]
It will not automatically remove anything, it just adds the new kernel to the system when the metapackage is updated, e.g.: