CA-87324: simulate VBD eject/insert as plug/unplug in xapi rather than xenopsd

Simulating this in xenopsd leads to states where a VDI has been
ejected but the device is still 'active' and therefore 'currently_attached=true'
Although this is self-consistent, it confuses xapi and the regression
tests using the XenAPI. Instead we fall back to transforming eject into
unplug and insert into plug at the XenAPI level.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>