qemu: Only one element of qemuPaths map is relevant

The qemuPaths field in qemuArchBase maps from machine type to the default
qemu path.  But, by the time we construct it, we already know the machine
type, so that entry ends up being the only one we care about.

So, collapse the map into a single path.  As a bonus, the qemuPath()
method can no longer fail.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

src/runtime/virtcontainers/qemu_test.go

@@ -346,9 +346,7 @@ func TestQemuQemuPath(t *testing.T) {
 			Type: "pc",
 			Options: "",
 		},
-		qemuPaths: map[string]string{
-			"pc": expectedPath,
-		},
+		qemuExePath: expectedPath,
 	}
 
 	q := &qemu{
@@ -372,13 +370,6 @@ func TestQemuQemuPath(t *testing.T) {
 	path, err = q.qemuPath()
 	assert.NoError(err)
 	assert.Equal(path, expectedPath)
-
-	// bad machine type, arch should fail
-	qkvm.qemuMachine.Type = "rgb"
-	q.arch = qkvm
-	path, err = q.qemuPath()
-	assert.Error(err)
-	assert.Equal(path, "")
 }
 
 func TestHotplugUnsupportedDeviceType(t *testing.T) {