Add a new "description" field to DevicePropertyInfo.
The descriptions can serve as documentation in the code,
and they can be used to provide better help. For example:

$./qemu-system-x86_64 -device virtio-blk-pci,?

Before this patch:

virtio-blk-pci.iothread=link<iothread>
virtio-blk-pci.x-data-plane=bool
virtio-blk-pci.scsi=bool
virtio-blk-pci.config-wce=bool
virtio-blk-pci.serial=str
virtio-blk-pci.secs=uint32
virtio-blk-pci.heads=uint32
virtio-blk-pci.cyls=uint32
virtio-blk-pci.discard_granularity=uint32
virtio-blk-pci.bootindex=int32
virtio-blk-pci.opt_io_size=uint32
virtio-blk-pci.min_io_size=uint16
virtio-blk-pci.physical_block_size=uint16
virtio-blk-pci.logical_block_size=uint16
virtio-blk-pci.drive=str
virtio-blk-pci.virtio-backend=child<virtio-blk-device>
virtio-blk-pci.command_serr_enable=on/off
virtio-blk-pci.multifunction=on/off
virtio-blk-pci.rombar=uint32
virtio-blk-pci.romfile=str
virtio-blk-pci.addr=pci-devfn
virtio-blk-pci.event_idx=on/off
vi...