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/pull.go

@@ -212,7 +212,7 @@ func Decompress(localPath, uncompressedPath string) error {
 	return decompressEverythingElse(localPath, uncompressedFileWriter)
 }
 
-// Will error out if file without .xz already exists
+// Will error out if file without .Xz already exists
 // Maybe extracting then renaming is a good idea here..
 // depends on Xz: not pre-installed on mac, so it becomes a brew dependency
 func decompressXZ(src string, output io.WriteCloser) error {