hw/ide/piix: Add some documentation to pci_piix3_xen_ide_unplug()

hw/ide/piix: Add some documentation to pci_piix3_xen_ide_unplug()

The comment is based on commit message
ae4d2eb273b167dad748ea4249720319240b1ac2 'xen-platform: add missing disk
unplug option'. Since it seems to describe design decisions and
limitations that still apply it seems worth having.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20220513180957.90514-3-shentey@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>