libxl: acquire a job when destroying a domain

libxl: acquire a job when destroying a domain

A job should be acquired at the beginning of a domain destroy operation,
not at the end when cleaning up the domain.  Fix two occurrences of this
late job acquisition in the libxl driver.  Doing so renders
libxlDomainCleanupJob unused, so it is removed.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>