enable discard option for dom0 filesystems by default

This may have performance impact on some older SSD, but on the other
hand, without this option it's pretty easy to fill the whole LVM thin
pool even if there is plenty free space in dom0.
Note that this doesn't enable it on LUKS layer, this is still disabled
by default.

Fixes QubesOS/qubes-issues#3226

pyanaconda/modules/storage/devicetree/fsset.py

@@ -776,6 +776,8 @@ def fstab(self):
                     break
             if device.encrypted:
                 options += ",x-systemd.device-timeout=0"
+            if fstype in ('ext4', 'btrfs', 'xfs', 'vfat'):
+                options += ",discard"
             devspec = device.fstab_spec
             dump = device.format.dump
             if device.format.check and mountpoint == "/":