machine refactoring preparations for hyperv

before we can support hyperv as a virtualization option for podman
machine, several areas in machine will require cleanup.  this is the
first pass of these changes to keep the review burden low.  changes
include:

  * convert artifact, format (image format) and compression to enums
    with string methods
  * rename Provider interface to VirtProvider
  * change Provider implementation in QEMU to QEMUVirt
  * change Provider implementation in WSL to WSLVirt

as mentioned earlier, there will be several more of these refactoring
PRs because assumptions were made about associations of platforms and
virt providers as well as compression and image formats.

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

pkg/machine/config.go

@@ -47,7 +47,7 @@ const (
 	DefaultMachineName string = "podman-machine-default"
 )
 
-type Provider interface {
+type VirtProvider interface {
 	NewMachine(opts InitOptions) (VM, error)
 	LoadVMByName(name string) (VM, error)
 	List(opts ListOptions) ([]*ListResponse, error)
@@ -72,10 +72,10 @@ var (
 
 type Download struct {
 	Arch                  string
-	Artifact              string
+	Artifact              artifact
 	CompressionType       string
 	CacheDir              string
-	Format                string
+	Format                imageFormat
 	ImageName             string
 	LocalPath             string
 	LocalUncompressedFile string