only set LIBGUESTFS_HV for ppc64le on el7

It appears the newer P9 hardware that we are utilizing that is running
a Fedora stack doesn't need this. Local testing seems to work fine.

Let's hack around this for now while we still have el7 clusters.

mantle/platform/qemu.go

@@ -50,6 +50,8 @@ import (
 	"github.com/coreos/mantle/system"
 	"github.com/coreos/mantle/system/exec"
 	"github.com/pkg/errors"
+
+	"golang.org/x/sys/unix"
 )
 
 var (
@@ -658,10 +660,20 @@ func newGuestfish(diskImagePath string, diskSectorSize int) (*coreosGuestfish, e
 	guestfishArgs = append(guestfishArgs, "-a", diskImagePath)
 	cmd := exec.Command("guestfish", guestfishArgs...)
 	cmd.Env = append(os.Environ(), "LIBGUESTFS_BACKEND=direct")
+
+	// Hack to run with a wrapper on older P8 hardware running RHEL7
 	switch system.RpmArch() {
 	case "ppc64le":
-		cmd.Env = append(cmd.Env, "LIBGUESTFS_HV=/usr/lib/coreos-assembler/libguestfs-ppc64le-wrapper.sh")
+		u := unix.Utsname{}
+		if err := unix.Uname(&u); err != nil {
+			return nil, errors.Wrapf(err, "detecting kernel information")
+		}
+		if strings.Contains(fmt.Sprintf("%s", u.Release), "el7") {
+			plog.Infof("Detected el7. Running using libguestfs-ppc64le-wrapper.sh")
+			cmd.Env = append(cmd.Env, "LIBGUESTFS_HV=/usr/lib/coreos-assembler/libguestfs-ppc64le-wrapper.sh")
+		}
 	}
+
 	// make sure it inherits stderr so we see any error message
 	cmd.Stderr = os.Stderr
 	stdout, err := cmd.StdoutPipe()

src/libguestfish.sh

@@ -11,8 +11,12 @@ export LIBGUESTFS_BACKEND=direct
 
 arch=$(uname -m)
 
+
+# Hack to run with a wrapper on older P8 hardware running RHEL7
 if [ "$arch" = "ppc64le" ] ; then
-    export LIBGUESTFS_HV="/usr/lib/coreos-assembler/libguestfs-ppc64le-wrapper.sh"
+    if [[ "$(uname -r)" =~ "el7" ]]; then
+        export LIBGUESTFS_HV="/usr/lib/coreos-assembler/libguestfs-ppc64le-wrapper.sh"
+    fi
 fi
 
 # http://libguestfs.org/guestfish.1.html#using-remote-control-robustly-from-shell-scripts