udev: don't exclude loop devices pointing outside of /var/lib/qubes

Generally list loop devices in qvm-block, but exclude only those
pointing at files in /var/lib/qubes (VM disk images).

Fixes QubesOS/qubes-issues#3084

system-config/00-qubes-ignore-devices.rules

@@ -1,6 +1,7 @@
 # do not edit this file, it will be overwritten on update
 
-ACTION!="remove", SUBSYSTEM=="block", KERNEL=="loop*|xvd*", ENV{DM_UDEV_DISABLE_DISK_RULES_FLAG}="1", ENV{UDEV_DISABLE_PERSISTENT_STORAGE_RULES_FLAG}="1"
+ACTION!="remove", SUBSYSTEM=="block", KERNEL=="loop*", ATTR{loop/backing_file}=="/var/lib/qubes/*", ENV{DM_UDEV_DISABLE_DISK_RULES_FLAG}="1", ENV{UDEV_DISABLE_PERSISTENT_STORAGE_RULES_FLAG}="1"
+ACTION!="remove", SUBSYSTEM=="block", KERNEL=="xvd*", ENV{DM_UDEV_DISABLE_DISK_RULES_FLAG}="1", ENV{UDEV_DISABLE_PERSISTENT_STORAGE_RULES_FLAG}="1"
 # ENV{DM_NAME} not available yet
 # Template VM disks
 ACTION!="remove", SUBSYSTEM=="block", ATTR{dm/name}=="snapshot-*", ENV{DM_UDEV_DISABLE_DISK_RULES_FLAG}="1"