Add support for RPMB over VirtIO Serial

In order to test Trusty gatekeeper automatically, the storage proxy
needs to be active inside the emulator. This patch allows storageproxyd
to speak a length-framed RPMB to an external RPMB daemon.

For a concrete example of a daemon speaking this protocol, see rpmb_dev
in the Trusty tree.

Bug: 124277696
Test: Launch storageproxyd with -t virt, use Trusty test infra
Change-Id: I391d4768976f0eb1f3b8df58eefd58fc3a9409cd