vcpu: improve support for setting xen vcpu counts

vcpu: improve support for setting xen vcpu counts

Tested with RHEL 5.6 (xendConfigVersion 2, where xend_internal
controls live domains and xm_internal controls inactive domains).
Hopefully this works with xendConfigVersion 3 (where xend_internal
controls everything).

* src/xen/xen_driver.c (xenUnifiedDomainSetVcpusFlags): Support
more flags.
(xenUnifiedGetMaxVcpus): Export.
* src/xen/xm_internal.h (xenXMDomainSetVcpusFlags): New prototype.
* src/xen/xend_internal.h (xenDaemonDomainSetVcpusFlags): Likewise.
* src/xen/xen_driver.h (xenUnifiedGetMaxVcpus): Likewise.
* src/xen/xm_internal.c (xenXMDomainSetVcpusFlags): New function.
* src/xen/xend_internal.c (xenDaemonDomainSetVcpusFlags): Likewise.