libxl: Introduce DOMAIN_DESTROYED error code

libxl: Introduce DOMAIN_DESTROYED error code

This is currently reported only by the bootloader code, if the domain
is destroyed while the bootloader is running.

In the future it would be nice to return it for other circumstances
where the domain existed when the operation started but subsequently
vanished.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
---
v4: Add a comment about the new error code's semantics
v2: New in this version of the series.