Also fix a logic bug in xenXMDomain{Attach,Detach}DeviceFlags,
where (flags & VIR_DOMAIN_DEVICE_MODIFY_CURRENT) is always false.

* src/xen/xen_driver.c (xenUnifiedDomainXMLFromNative)
(xenUnifiedDomainXMLToNative, xenUnifiedDomainBlockPeek): Reject
unknown flags.
* src/xen/xen_hypervisor.c (xenHypervisorOpen)
(xenHypervisorGetDomainState): Likewise.
* src/xen/xen_inotify.c (xenInotifyOpen): Likewise.
* src/xen/xs_internal.c (xenStoreOpen, xenStoreDomainGetState)
(xenStoreDomainReboot): Likewise.
* src/xen/xend_internal.c (xenDaemonOpen, xenDaemonDomainReboot)
(xenDaemonDomainCoreDump, xenDaemonDomainGetState)
(xenDaemonDomainMigratePrepare, xenDaemonDomainSetVcpusFlags,
xenDaemonDomainGetVcpusFlags, xenDaemonAttachDeviceFlags,
xenDaemonDetachDeviceFlags): Likewise.
(xenDaemonDomainGetXMLDesc): Prefer unsigned flags.
* src/xen/xend_internal.h (xenDaemonDomainGetXMLDesc): Likewise.
* src/xen/xm_internal.h (xenXMDomainGetXMLDesc): Likewise.
* src/xen/xm_internal.c (xenXMDomainGetXMLDesc): Likewise.
(xenXMOpen, xenXMDomainGetState, xenXMDomainSetVcpusFlags)
(xenXMDomainGetVcpusFlags): Reject unknown flags.
(xenXMDomainAttachDeviceFlags, xenXMDomainDetachDeviceFlags):
Likewise, and avoid always-false conditional.
* src/xen/xen_driver.h (XEN_MIGRATION_FLAGS): New define.