x86/HVM: patch vINTR indirect calls through hvm_funcs to direct ones

x86/HVM: patch vINTR indirect calls through hvm_funcs to direct ones

While not strictly necessary, change the VMX initialization logic to
update the function table in start_vmx() from NULL rather than to NULL,
to make more obvious that we won't ever change an already (explicitly)
initialized function pointer.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>