Mount /dev when creating a VM image from RPMs

Currently, using `runInLinuxVM` fails for CentosOS7 versions >=7.4. The
error manifests itself by the VM powering down after failing to
initialize the NSS library. The reason for this is that some devices in
/dev are required during the built process of the VM.

To fix this issue, we mount /dev before calling chroot.

pkgs/build-support/vm/default.nix

@@ -449,6 +449,8 @@ rec {
         # Make the Nix store available in /mnt, because that's where the RPMs live.
         mkdir -p /mnt${storeDir}
         ${util-linux}/bin/mount -o bind ${storeDir} /mnt${storeDir}
+        # Some programs may require devices in /dev to be available (e.g. /dev/random)
+        ${util-linux}/bin/mount -o bind /dev /mnt/dev
 
         # Newer distributions like Fedora 18 require /lib etc. to be
         # symlinked to /usr.
@@ -487,7 +489,7 @@ rec {
 
         rm /mnt/.debug
 
-        ${util-linux}/bin/umount /mnt${storeDir} /mnt/tmp ${lib.optionalString unifiedSystemDir "/mnt/proc"}
+        ${util-linux}/bin/umount /mnt${storeDir} /mnt/tmp /mnt/dev ${lib.optionalString unifiedSystemDir "/mnt/proc"}
         ${util-linux}/bin/umount /mnt
       '';