Add utility to convert VMFile to URL for UNIX sockets

This adds generic utility to convert file system path into URL structure.
Instead of string manipulation it uses URL parsing and building routines.
Appending absolute path to `unix:///` URL out of the box correctly
handles URL format on Windows platform, where filepath should be prepended
by additional `/` before drive letter.

Signed-off-by: Arthur Sengileyev <arthur.sengileyev@gmail.com>

pkg/machine/qemu/stubber.go

@@ -9,7 +9,6 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
-	"path/filepath"
 	"strconv"
 	"strings"
 	"time"
@@ -319,11 +318,15 @@ func (q *QEMUStubber) StartNetworking(mc *vmconfigs.MachineConfig, cmd *gvproxy.
 	if err != nil {
 		return err
 	}
+	socketURL, err := sockets.ToUnixURL(gvProxySock)
+	if err != nil {
+		return err
+	}
 	// make sure it does not exist before gvproxy is called
 	if err := gvProxySock.Delete(); err != nil {
 		logrus.Error(err)
 	}
-	cmd.AddQemuSocket(fmt.Sprintf("unix://%s", filepath.ToSlash(gvProxySock.GetPath())))
+	cmd.AddQemuSocket(socketURL.String())
 	return nil
 }