udev: don't call udev-block-add-change for devices excluded by other rules

The script call is quite expensive (it does multiple things, including
checking device-mapper, qubesdb etc). Don't call it for devices we (or
else) already excluded earlier.
This is the most relevant for dom0, where udev "change" event is
triggered quite often, for multiple LVM volumes - all excluded, because
being VM's disks.

udev/udev-qubes-block.rules

@@ -10,6 +10,9 @@ KERNEL=="xvda|xvdb|xvdc*|xvdd", ENV{UDISKS_IGNORE}="1"
 # Skip xen-blkfront devices
 ENV{MAJOR}=="202", GOTO="qubes_block_end"
 
+# skip devices excluded elsewhere
+ENV{DM_UDEV_DISABLE_DISK_RULES_FLAG}=="1", GOTO="qubes_block_end"
+
 # Skip device-mapper devices
 KERNEL=="dm-*", ENV{DM_NAME}=="snapshot-*", GOTO="qubes_block_end"
 KERNEL=="dm-*", ENV{DM_NAME}=="origin-*", GOTO="qubes_block_end"