docs: update HVM emulated unplug protocol to cover NVMe disks

docs: update HVM emulated unplug protocol to cover NVMe disks

Recent discussions on xen-devel have highlighted that to properly
support displacing emulated NVMe disks with PV equivalents will need
updates to PV frontends. Therefore it is important that, if an emulated
NVMe disk is exposed to a guest with an existing PV storage frontend,
that frontend does not inadvertently cause unplug of that emulated
disk when unplugging IDE or SCSI disks.

This patch defines a new bit in the mask used to instruct QEMU to unplug
emulated devices which will instruct QEMU to unplug NVMe disks and limits
the semantics of the existing 'all' disk-unplug bit to only IDE and/or SCSI
disks.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>