Add the pvscsi driver to the tree.

This driver allows to usage of the paravirt SCSI controller
in VMware products like ESXi. The pvscsi driver provides a
substantial performance improvement in block devices versus
the emulated mpt and mps SCSI/SAS controllers.

Error handling in this driver has not been extensively tested
yet.

Submitted by: vbhakta@vmware.com
Relnotes: yes
Sponsored by: VMware, Panzura
Differential Revision: D18613