libxl: disks: rename disk param "unpluggable" to "removable"

libxl: disks: rename disk param "unpluggable" to "removable"

This property corresponds to what is called "removable" in xenstore,
and is the conventional meaning of "removable": ie, the _media_ can be
removed even as the _device_ remains present.

"unpluggable" is a misleading name for this.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>