CHANGELOG: add env changes

### environment

spdk_vtophys() has been refactored to accept length of the translated region as a new

parameter. The function will now update that parameter with the largest possible value

for which the memory is contiguous in the physical memory address space.

The following functions were removed:

 - spdk_pci_nvme_device_attach()

 - spdk_pci_nvme_enumerate()

 - spdk_pci_ioat_device_attach()

 - spdk_pci_ioat_enumerate()

 - spdk_pci_virtio_device_attach()

 - spdk_pci_virtio_enumerate()

They were replaced with generic spdk_pci_device_attach() and spdk_pci_enumerate() which

require a new spdk_pci_driver object to be provided. It can be one of the following:

 - spdk_pci_nvme_get_driver()

 - spdk_pci_ioat_get_driver()

 - spdk_pci_virtio_get_driver()

spdk_pci_hook_device() and spdk_pci_unhook_device() were added. Those allow adding a virtual

spdk_pci_device into the SPDK PCI subsystem. A virtual device calls provided callbacks for

each BAR mapping request or PCI config access. It's attachable with spdk_pci_device_attach()

or spdk_pci_enumerate() like any other device.

A new spdk_pause() function was added to pause CPU execution for an implementation specific

amount of time. Quoting from DPDK function this is based on: "This call is intended for

tight loops which poll a shared resource or wait for an event. A short pause within the loop

may reduce the power consumption."

A new public header file env_dpdk.h has been introduced, and function spdk_env_dpdk_post_init

is added into it. If user is using DPDK, and already called rte_eal_init, then include

include/spdk/env_dpdk.h, and call spdk_env_dpdk_post_init() instead of spdk_env_init.