Support emulator's virtio-blk based SD card

Support emulator's virtio-blk based SD card

Currently, vold only supports MMC (for SD cards) and SCSI (for USB
drives) devices. It does not recognize any device whose major number is
not one of those used by MMC and SCSI. Unfortunately, virtio-blk is one
such device. It is used by the new Android emulator (a.k.a. qemu2,
featuring the "ranchu" virtual board) for SD card emulation.

In order to make this virtio-blk based SD card device appear in Android
and appear as an SD card (rather than a USB drive), changes have to be
made to both vold (wherever the device major number is checked) and
ranchu's storage configuration. This CL implements former.

This is a stop-gap solution for emulator in nyc.
A longer term solution in-tune with upstream kernel is in the pipes.

Updated from aosp/master version.

BUG:27431753

Change-Id: I5014edec73be7c5b565d91542464c82cbe58992c
Signed-off-by: Yu Ning <yu.ning@intel.com>
(cherry picked from commit 5b1d1c7dfa13b4dca75213581dc8351b841b76c8)