libxl: abort bootloader invocation when domain dies

libxl: abort bootloader invocation when domain dies

Cancel the bootloader (specifically, by sending it a signal) if the
domain is seen to disappear from xenstore.

We use a new utility event source libxl__domaindeathcheck which
provides a convenient wrapper for the xenstore watch.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Changes since v8:
 * Fixed the commit message summary line.

Changes since v7:
 * Add a comment explaining why we use a watch on the domain's
   xenstore path rather than @releaseDomain.
 * Fix typo in error message.
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>