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
5 files changed