feat(qemu): include the virtio_mem kernel module

feat(qemu): include the virtio_mem kernel module

This adds support for virtio-mem devices, which provide a dynamic
amount of memory in a VM. Right now, the driver gets loaded and any
memory gets added to the system when loading the kernel module from disk.

While not strictly required to boot, we want to be able to
1) add virito-mem provided memory to the system early while booting up
2) add virtio-mem provided memory even when booting without a disk
3) add virtio-mem devices without adding actual memory in kdump
   environments such that we can query things like:
 a) is a certain PFN currently plugged in the hypervisor and, therefore,
    should actually be read when creating a system dump. (kexec-tools
    prepares the vmcore header, like on x86-64)
 b) which ranges of a virtio-mem device are currently plugged in the
    hypervisor and, therefore, should be added to the dump. (vmcore header
    gets prepared by the crashkernel, like on s390x)
 Note that loading virtio-mem in kdump environments currently fails with
 -EBUSY -- but there are plans to install proper hooks instead to support
  especially a) in the near future.

1) and 2) are only really effective when memory hotplug is configured to
automatically online all added system RAM in the kernel (and not late,
via udev rules): e.g., via "mhp_default_state=online" on the kernel
cmdline or via CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE in the kernel.

Especially 2) and 3) require the module to be present inside the initial
ramdisk. The primary use case for including it in the initial ramdisk
is 3).

Signed-off-by: David Hildenbrand <david@redhat.com>