fake images: windows hyperv

this pr is a follow on to #27493.  it adds support for hyperv "fake"
images and suggests a benefit in terms of test speed.  for hyperv, we
create a generic 4MB vhdx and stick it into the temp dir.  this saves us
from any image copy or compression.

i also followed up on a few comments Paul made about using windows|unix
instead of each platform.

Signed-off-by: Brent Baude <bbaude@redhat.com>

pkg/machine/e2e/config_unix_test.go

@@ -3,10 +3,24 @@
 package e2e_test
 
 import (
+	"os"
 	"os/exec"
 )
 
+var fakeImagePath string = os.DevNull
+
 func pgrep(_ string) (string, error) {
 	out, err := exec.Command("pgrep", "gvproxy").Output()
 	return string(out), err
 }
+
+func initPlatform()    {}
+func cleanupPlatform() {}
+
+// withFakeImage should be used in tests where the machine is
+// initialized (or not) but never started.  It is intended
+// to speed up CI by not processing our large machine files.
+func (i *initMachine) withFakeImage(_ *machineTestBuilder) *initMachine {
+	i.image = fakeImagePath
+	return i
+}