urlfetcher: Don't set os_variant=linux

os_variant=linux is basically meaningless; it doesn't set any useful
defaults. better to be explicit that we didn't detect any meaningful
os_variant, to trigger the virt-install warning

tests/cli-test-xml/compare/virt-install-test-url-detection.xml

@@ -1 +1 @@
-linux
+None

tests/test_urls.py

@@ -67,7 +67,7 @@ _distro = None
 
 
 class _DistroURL(object):
-    def __init__(self, x86_64, detectdistro="linux", i686=None,
+    def __init__(self, x86_64, detectdistro=None, i686=None,
                  hasxen=True, hasbootiso=True, name=None,
                  testshortcircuit=False):
         self.x86_64 = x86_64
@@ -174,7 +174,7 @@ _add(UBUNTU_URL % ("wily", "amd64"), "ubuntu15.10")
 
 
 _set_distro(MandrivaDistro)
-_add(MAGEIA_URL % ("5", "x86_64"), "linux", name="mageia5", hasxen=False)
+_add(MAGEIA_URL % ("5", "x86_64"), name="mageia5", hasxen=False)
 
 
 testconn = utils.open_testdefault()

virtinst/urlfetcher.py

@@ -661,7 +661,6 @@ class GenericDistro(Distro):
     as a last resort if we can't recognize any actual distro
     """
     name = "Generic"
-    os_variant = "linux"
     uses_treeinfo = True
 
     _xen_paths = [("images/xen/vmlinuz",
@@ -753,7 +752,6 @@ class RedHatDistro(Distro):
     a common layout
     """
     uses_treeinfo = True
-    os_variant = "linux"
     _version_number = None
 
     _boot_iso_paths   = ["images/boot.iso"]
@@ -1068,7 +1066,6 @@ class DebianDistro(Distro):
     # daily builds: http://d-i.debian.org/daily-images/amd64/
     name = "Debian"
     urldistro = "debian"
-    os_variant = "linux"
 
     def __init__(self, *args, **kwargs):
         Distro.__init__(self, *args, **kwargs)
@@ -1185,7 +1182,6 @@ class MandrivaDistro(Distro):
     # ftp://ftp.uwsg.indiana.edu/linux/mandrake/official/2007.1/x86_64/
     name = "Mandriva/Mageia"
     urldistro = "mandriva"
-    os_variant = "linux"
 
     _boot_iso_paths = ["install/images/boot.iso"]
     _xen_kernel_paths = []
@@ -1228,7 +1224,6 @@ class ALTLinuxDistro(Distro):
     # mounted ISO
     name = "ALT Linux"
     urldistro = "altlinux"
-    os_variant = "linux"
 
     _boot_iso_paths = [("altinst", "live")]
     _hvm_kernel_paths = [("syslinux/alt0/vmlinuz", "syslinux/alt0/full.cz")]