A destroy operation can take considerable time on large memory
domains due to scrubbing the domain's memory.  Unlock the
virDomainObj while libxl_domain_destroy is executing.

Implement libxlDomainDestroyInternal wrapper to handle unlocking,
calling destroy, and locking.  Change all callers of
libxl_domain_destroy to use the wrapper.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>