The Intel VMD (Volume Management Device) driver is explained by the new Kconfig entry as follows, "VMD is a secondary PCI host bridge that allows PCI Express root ports, and devices attached to them, to be removed from the default PCI domain and placed within the VMD domain." And it's further explained by its original patch series.
The Intel VMD driver landed alongside other PCI changes for Linux 4.5. Some of the other PCI work includes Broadcom iProc driver improvements, support for the HiSilicon Hip06 PCIe host controllers, a new Qualcomm PCI Express controller driver, support for the R-Car H3 in the pcie-rcar driver, and runtime power management for the R-Car driver.
There are also a number of bug-fixes and core improvements to the PCI code as well as some of the other individual PCI/PCI-E drivers. More details on the PCI changes for Linux 4.5 can be found via this pull request from earlier in the week.