None of the callers cared if str was updated to point to the next
byte after the parsed cpuset; simplifying this results in quite
a few code simplifications.  Additionally, virCPUDefParseXML was
strdup()'ing a malloc()'d string; avoiding a memory copy resulted
in less code.

* src/conf/domain_conf.h (virDomainCpuSetParse): Alter signature.
* src/conf/domain_conf.c (virDomainCpuSetParse): Don't modify str.
(virDomainVcpuPinDefParseXML, virDomainDefParseXML): Adjust
callers.
* src/conf/cpu_conf.c (virCPUDefParseXML): Likewise.
* src/xen/xend_internal.c (sexpr_to_xend_topology): Likewise.
* src/xen/xm_internal.c (xenXMDomainPinVcpu): Likewise.
* src/xenxs/xen_sxpr.c (xenParseSxpr): Likewise.
* src/xenxs/xen_xm.c (xenParseXM): Likewise.