anaconda: use proper subvolume argument when booting from btrfs (EFI)

Kernel command line in legacy mode is constructed by grub scripts and
properly handle btrfs subvolumes. For EFI, it is built directly by
anaconda and 'rootflags=subvol=...' argument need to be added manually.

Fixes QubesOS/qubes-issues#1871

pyanaconda/bootloader.py

@@ -1973,12 +1973,15 @@ def write_config_header(self, config):
 
     def write_config_images(self, config):
         for image in self.images:
+            root_args = 'root=' + image.device.fstab_spec
+            if image.device.type == "btrfs subvolume":
+                root_args += " rootflags=subvol=%s" % image.device.name
             config.write("\n")
             config.write("[{}]\n".format(image.version))
             config.write("options=loglvl=all dom0_mem=min:1024M dom0_mem=max:4096M iommu=no-igfx\n")
-            config.write("kernel={} root={} {}\n".format(
+            config.write("kernel={} {} {}\n".format(
                 image.kernel,
-                image.device.fstab_spec,
+                root_args,
                 self.boot_args))
             config.write("ramdisk={}\n".format(image.initrd))