There an issue filled against libvirt that basically boils down
to not enough memory being set by default for Gentoo live ISO.
No surprise there - we claim the minimal requirement is 256MiB
(!). And live ISO starts a KDE session on boot.

Now, it's hard to come up with something that'll work in 100%
cases and still be minimal. There's this recommendation in
Gentoo's handbook [1], but the page was last edited in 2014.

Anyway, I've tried to boot the live ISO and the VM was using
~1.5GiB of memory. So let's put the minimum as 2GiB as it could
grant enough memory for emerge/compilation or basic system tools.

And while at it, recommend 4GiB of RAM and 20GiB of storage.
There are packages that require much more, e.g.
www-client/chrominum may require up to 16GiB [2], but that's
rather exception. And recently, Gentoo added support for binary
packages too which helps to save some memory as users don't need
to compile packages themselves.

1: https://wiki.gentoo.org/wiki/Handbook:X86/Full/Installation#Hardware_requirements
2: https://github.com/gentoo/gentoo/blob/master/www-client/chromium/chromium-125.0.6422.14.ebuild#L352
Resolves: https://gitlab.com/libvirt/libvirt/-/issues/600
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>