Cleanup after a previous patch, commit 6e130ddc.  In particular,
note that xenDomainUsedCpus can only be reached from
xenUnifiedDomainGetXMLDesc, which in turn is only reached from
public API that already validated the domain.

* src/xen/xen_driver.c (xenDomainUsedCpus): Drop redundant check.
* src/datatypes.h (VIR_IS_DOMAIN, VIR_IS_CONNECTED_DOMAIN):
Delete, and inline into all callers, since no other file uses it
any more.
Signed-off-by: NEric Blake <eblake@redhat.com>