vfkit: Don't try to use virtiofs it's unsupported

virtiofs file sharing is only available in macOS 12 and newer. This
commit makes use of sw_vers to return a proper error when running on too
old macOS versions.
This also skips vfkit shared directory configuration so that we can
start the VM anyway.
For all of this to work, vfkit has to be built on a macOS 11 builder,
the file sharing APIs will still be available when this binary is run on
macOS 12.
Building vfkit on macOS 12 and trying to run the resulting binary on
macOS 11 will badly fail though:

```
DEBU Running '/Users/teuf/.crc/bin/vfkit -v'
DEBU Command failed: signal: abort trap
DEBU stdout:
DEBU stderr: dyld: Symbol not found:
_OBJC_CLASS_$_VZMultipleDirectoryShare
  Referenced from: /Users/teuf/.crc/bin/vfkit (which was built for Mac OS X 12.0)
  Expected in:
/System/Library/Frameworks/Virtualization.framework/Versions/A/Virtualization
 in /Users/teuf/.crc/bin/vfkit
DEBU failed to run executable /Users/teuf/.crc/bin/vfkit: signal: abort trap
```

pkg/drivers/vfkit/driver_darwin.go

@@ -202,16 +202,18 @@ func (d *Driver) Start() error {
 	}
 
 	// shared directories
-	for _, sharedDir := range d.SharedDirs {
-		// TODO: add support for 'mount.ReadOnly'
-		// TODO: check format
-		dev, err := client.VirtioFsNew(sharedDir.Source, sharedDir.Tag)
-		if err != nil {
-			return err
-		}
-		err = vm.AddDevice(dev)
-		if err != nil {
-			return err
+	if d.supportsVirtiofs() {
+		for _, sharedDir := range d.SharedDirs {
+			// TODO: add support for 'mount.ReadOnly'
+			// TODO: check format
+			dev, err := client.VirtioFsNew(sharedDir.Source, sharedDir.Tag)
+			if err != nil {
+				return err
+			}
+			err = vm.AddDevice(dev)
+			if err != nil {
+				return err
+			}
 		}
 	}
 
@@ -279,6 +281,9 @@ func (d *Driver) Start() error {
 }
 
 func (d *Driver) GetSharedDirs() ([]drivers.SharedDir, error) {
+	if !d.supportsVirtiofs() {
+		return nil, drivers.ErrNotSupported
+	}
 	// check if host supports file sharing, return drivers.ErrNotSupported if not
 	return d.SharedDirs, nil
 }
@@ -461,3 +466,11 @@ func (d *Driver) sendSignal(s syscall.Signal) error {
 
 	return proc.SendSignal(s)
 }
+
+func (d *Driver) supportsVirtiofs() bool {
+	macosVersion, _, err := crcos.RunWithDefaultLocale("sw_vers", "-productVersion")
+	if err != nil {
+		return false
+	}
+	return strings.HasPrefix(macosVersion, "12.")
+}