bhyve: implement NVMe device support

bhyve: implement NVMe device support

NVMe devices in bhyve are modeled this way:

 -s $pciaddr,nvme,devpath[,opts]

devpath can be a path to the image or the block device. It also can be
"ram=size_in_MiB", but this is not covered by this series.

There could be only a single device per PCI address.

Optional configuration options (such as max number of queues, concurrent
I/O requests, etc) are also not covered by this series.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>