udev: ignore devices set to be ignore elsewhere

There are already some other rules to ignore not interesting devices.
This includes device-mapper assembled in initramfs manually. 'dmroot'
isn't properly detected as mounted because /dev/mapper/dmroot isn't a
symlink to /dev/dm-0 and /proc/mounts contains the former name, while
udev event the later.

Fixes QubesOS/qubes-issues#1586

udev/udev-qubes-block.rules

@@ -14,6 +14,7 @@ ENV{MAJOR}=="202", GOTO="qubes_block_end"
 KERNEL=="dm-*", ENV{DM_NAME}=="snapshot-*", GOTO="qubes_block_end"
 KERNEL=="dm-*", ENV{DM_NAME}=="origin-*", GOTO="qubes_block_end"
 KERNEL=="dm-*", ENV{DM_NAME}=="", GOTO="qubes_block_end"
+ENV{DM_UDEV_DISABLE_DISK_RULES_FLAG}=="1", GOTO="qubes_block_end"
 
 IMPORT{db}="QUBES_EXPOSED"
 ACTION=="add", IMPORT{program}="/usr/lib/qubes/udev-block-add-change"