Make /dev/kvm optional

The daemon now creates /dev deterministically (thanks!).  However, it
expects /dev/kvm to be present.

The patch below restricts that requirement (1) to Linux-based systems,
and (2) to systems where /dev/kvm already exists.

I’m not sure about the way to handle (2).  We could special-case
/dev/kvm and create it (instead of bind-mounting it) in the chroot, so
it’s always available; however, it wouldn’t help much since most likely,
if /dev/kvm missing, then KVM support is missing.

src/libstore/build.cc

@@ -2020,7 +2020,10 @@ void DerivationGoal::initChild()
                 createDirs(chrootRootDir + "/dev/pts");
                 Strings ss;
                 ss.push_back("/dev/full");
+#ifdef __linux__
+                if (pathExists("/dev/kvm"))
                     ss.push_back("/dev/kvm");
+#endif
                 ss.push_back("/dev/null");
                 ss.push_back("/dev/random");
                 ss.push_back("/dev/tty");