Add VMCI (Virtual Machine Communication Interface) driver

In a virtual machine, VMCI is exposed as a regular PCI device. The primary
communication mechanisms supported are a point-to-point bidirectional
transport based on a pair of memory-mapped queues, and asynchronous
notifications in the form of datagrams and doorbells. These features are
available to kernel level components such as vSockets through the VMCI
kernel API. In addition to this, the VMCI kernel API provides support for
receiving events related to the state of the VMCI communication channels,
and the virtual machine itself.

Submitted by: Vishnu Dasa <vdasa@vmware.com>
Reviewed by: bcr, imp
Obtained from: VMware
Differential Revision: https://reviews.freebsd.org/D14289