udev/qvm-block: exclude device if its partition is already attached

This prevents simultaneous attachment of the same device (in practice)
to multiple VMs.
This change to be effective requires udev action being called when such
device is attached/detached to some domain. Script
/etc/xen/scripts/block will take care of it.

Fixes QubesOS/qubes-issues#1081

udev/udev-block-add-change

@@ -36,11 +36,21 @@ is_used() {
     return 1
 }
 
+is_attached() {
+    dev_hex=$(stat -c %t:%T /dev/$(basename $1))
+    xenstore-ls backend/vbd | grep -q "physical-device = \"$dev_hex\""
+}
+
 # update info about parent devices, if any:
 if [ -f /sys$DEVPATH/partition ]; then
     parent=$(dirname $(readlink -f /sys$DEVPATH))
     udevadm trigger \
         --property-match=DEVPATH=/$(realpath --relative-to=/sys $parent)
+    # if parent device is already attached, skip its partitions
+    if is_attached $parent; then
+        xs_remove
+        exit 0
+    fi
 fi
 
 # and underlying devices of device-mapper (if any)
@@ -58,12 +68,18 @@ if is_used /sys$DEVPATH; then
 fi
 
 # or one of its partitions is used
+# or already attached (prevent attaching both device and its partition(s) at
+# the same time)
 for part in /sys$DEVPATH/$NAME*; do
     if [ -d $part ]; then
         if is_used $part; then
             xs_remove
             exit 0
         fi
+        if is_attached $part; then
+            xs_remove
+            exit 0
+        fi
     fi
 done