tools/libxc: Avoid cacheflush toolstack hypercalls on x86

tools/libxc: Avoid cacheflush toolstack hypercalls on x86

XEN_DOMCTL_cacheflush hypercalls are (and will always be) -ENOSYS on x86, but
xc_domain_cacheflush() is called often during domain build and migrate for
correct behaviour on ARM.

Stub xc_domain_cacheflush() out on x86 to remove its pressure on the global
domctl lock, and the hypercall overhead (which applies further pressure to the
already heavily-contended TLB flush lock).

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Tim Deegan <tim@xen.org>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>