MFC r269814:
MFC r269814:

blkfront: add support for unmapped IO

Sponsored by:	Citrix Systems R&D
Tested by:	robak
PR:		191173

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase