Sort guest plugins before running detection

When detecting the machine guest, first sort the guest plugins
    by the number of ancestors. This allows for returning on the
    first match instead of requiring running the detection process
    on every registered guest plugin.

internal/core/machine.go

@@ -3,6 +3,7 @@ package core
 import (
 	"fmt"
 	"reflect"
+	"sort"
 
 	"github.com/golang/protobuf/ptypes"
 	"github.com/mitchellh/mapstructure"
@@ -112,12 +113,23 @@ func (m *Machine) Guest() (g core.Guest, err error) {
 		return
 	}
 
-	var result core.Guest
-	var result_name string
-	var numParents int
+	names := make([]string, 0, len(guests))
+	pcount := map[string]int{}
 
 	for name, g := range guests {
-		guest := g.Value.(core.Guest)
+		names = append(names, name)
+		pcount[name] = g.plugin.ParentCount()
+	}
+
+	sort.Slice(names, func(i, j int) bool {
+		in := names[i]
+		jn := names[j]
+		// TODO check values exist before use
+		return pcount[in] > pcount[jn]
+	})
+
+	for _, name := range names {
+		guest := guests[name].Value.(core.Guest)
 		detected, err := guest.Detect(m.toTarget())
 		if err != nil {
 			m.logger.Error("guest error on detection check",
@@ -127,41 +139,17 @@ func (m *Machine) Guest() (g core.Guest, err error) {
 
 			continue
 		}
-		if result == nil {
-			if detected {
-				result = guest
-				result_name = name
-				numParents = g.plugin.ParentCount()
-			}
-			continue
-		}
-
 		if detected {
-			gp := g.plugin.ParentCount()
-
-			if gp > numParents {
-				result = guest
-				result_name = name
-				numParents = gp
-			}
+			m.logger.Info("guest detection complete",
+				"name", name,
+			)
+			m.seedPlugin(guest)
+			m.guest = guest
+			return guest, nil
 		}
 	}
 
-	if result == nil {
-		return nil, fmt.Errorf("failed to detect guest plugin for current platform")
-	}
-
-	m.logger.Info("guest detection complete",
-		"name", result_name)
-
-	// NOTE: For guest seeding we need to prevent guest plugin instance
-	// from being cached and reused. Currently, in a multi-machine setup
-	// which are the same guest, the target values will get appended
-	// TODO(spox): Fix this in the plugin manager
-	m.seedPlugin(result)
-	m.guest = result
-	g = result
-	return result, nil
+	return nil, fmt.Errorf("failed to detect guest plugin for current platform")
 }
 
 func (m *Machine) seedPlugin(plg interface{}) (err error) {