Allow apexd to access a new dev_type: virtual disk

In microdroid, apexd activates apexes which are passed as a virtual disk
to share apexes with host Android.

Bug: 184605708
Test: apexd running in microdroid can read /dev/block/vdb2
  when a disk image is passed to crosvm via --disk= option.
Change-Id: Ie27774868a0e0befb4c42cff795d1531b042654c
diff --git a/private/file_contexts b/private/file_contexts
index 3786147..3a8642d 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -88,6 +88,7 @@
 /dev/block(/.*)?	u:object_r:block_device:s0
 /dev/block/dm-[0-9]+	u:object_r:dm_device:s0
 /dev/block/loop[0-9]*	u:object_r:loop_device:s0
+/dev/block/vd[a-z][0-9]*  u:object_r:vd_device:s0
 /dev/block/vold/.+	u:object_r:vold_device:s0
 /dev/block/ram[0-9]*	u:object_r:ram_device:s0
 /dev/block/zram[0-9]*	u:object_r:ram_device:s0