OVMF: Add Block MMIO protocol definition

OVMF: Add Block MMIO protocol definition

This protocol is similar to the standard UEFI BlockIo protocol,
except it has no function calls and simply defines a base address
in memory where reads & writes for the block device should occur.

One planned usage is to fill a memory region with a small disk
image, and allow it to be used as a normal disk by the
standard drivers.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10295 6f19259b-4bc3-4df7-8a09-765794883524