Increase the installer RAM amount to 8GiB.

With 4GiB guix pull fails. With 8GiB 'df -h /' returns the following:
    none            3.9G  1.6G  2.4G  40% /
so if we don't change the ramfs mount options it is pretty clear that
4GiB is not enough.

The issue here is that we are trying to do an install with the
following constraints:
- It needs to work without external access to the underlying
  storage. For instance I cannot become root and simply copy the image
  to the correct location and boot that because I lack the permissions
  but also because the machine doesn't have Guix.
- I also want the target rootfs to be up to date (especially with
  security updates) from the start.
- It needs to be relatively fast and not consume too much resources.
- It also needs to be relatively easy to deploy. Using the guix
  installer to install Guix is the easiest as it works out of the box
  with a read-only install media, but it also comes with resources
  costs as the files being created end up in RAM.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>

guix-vm-installer.xml

@@ -1,7 +1,7 @@
 <domain type="kvm">
   <name>guix-vm-installer</name>
-  <memory unit="KiB">4194304</memory>
-  <currentMemory unit="KiB">4194304</currentMemory>
+  <memory unit="KiB">16777216</memory>
+  <currentMemory unit="KiB">16777216</currentMemory>
   <resource>
     <partition>/machine</partition>
   </resource>