x86/hvm: Remove callback from paging->flush_tlb() hook

x86/hvm: Remove callback from paging->flush_tlb() hook

TLB flushing is a hotpath, and function pointer calls are
expensive (especially under retpoline) for what amounts to an identity
transform on the data.  Just pass the vcpu_bitmap bitmap directly.

As we use NULL for all rather than none, introduce a flush_vcpu() helper to
avoid the risk of logical errors from opencoding the expression.  This also
means the viridian callers can avoid writing an all-ones bitmap for the
flushing logic to consume.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>