The Android Emulator isn't the only virtual device the virtio-block
detection code is useful for, and those platforms might not set any
discriminating properties to indicate that they are virtual.
Rework the virtio-block major detection to use /proc/devices instead
of hardcoding the assumption that any virtual platform can have
virtio-block at any experimental major; the new code permits only the
exact experimental major assigned to virtio-block.
The new code runs everywhere, but it will only run once and could be
expanded later to detect dynamic or experimental majors.
Bug: 156286088
Change-Id: Ieae805d08fddd0124a397636f04d99194a9ef7e5
ff1fc9bc41