qemu: use 2048 MiB on all architectures

The alternatives test is now failing because rpm-ostree gets OOM
killed because we're now including the coreos-pool repo when layering
iptables-legacy [1].

Let's just move x86_64 to the default of 2048 that we're using
everywhere else too.

[1] coreos/fedora-coreos-config#4038

Author: dustymabe

mantle/platform/qemu.go

@@ -1366,24 +1366,7 @@ func (builder *QemuBuilder) finalize() {
 		return
 	}
 	if builder.MemoryMiB == 0 {
-		// FIXME; Required memory should really be a property of the tests, and
-		// let's try to drop these arch-specific overrides.  ARM was bumped via
-		// commit 09391907c0b25726374004669fa6c2b161e3892f
-		// Commit:     Geoff Levand <geoff@infradead.org>
-		// CommitDate: Mon Aug 21 12:39:34 2017 -0700
-		//
-		// kola: More memory for arm64 qemu guest machines
-		//
-		// arm64 guest machines seem to run out of memory with 1024 MiB of
-		// RAM, so increase to 2048 MiB.
-
-		// Then later, other non-x86_64 seemed to just copy that.
-		memory := 1024
-		switch builder.architecture {
-		case "aarch64", "s390x", "ppc64le":
-			memory = 2048
-		}
-		builder.MemoryMiB = memory
+		builder.MemoryMiB = 2048
 	}
 	builder.finalized = true
 }