x86/HVM: properly reject "indirect" VRAM writes

While ->count will only be different from 1 for "indirect" (data in
guest memory) accesses, it being 1 does not exclude the request being an
"indirect" one. Check both to be on the safe side, and bring the ->count
part also in line with what ioreq_send_buffered() actually refuses to
handle.

Fixes: 3bbaaec09b ("x86/hvm: unify stdvga mmio intercept with standard mmio intercept")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/hvm/stdvga.c

@@ -498,13 +498,13 @@ static bool cf_check stdvga_mem_accept(
 
     spin_lock(&s->lock);
 
-    if ( p->dir == IOREQ_WRITE && p->count > 1 )
+    if ( p->dir == IOREQ_WRITE && (p->data_is_ptr || p->count != 1) )
     {
         /*
          * We cannot return X86EMUL_UNHANDLEABLE on anything other then the
          * first cycle of an I/O. So, since we cannot guarantee to always be
          * able to send buffered writes, we have to reject any multi-cycle
-         * I/O.
+         * or "indirect" I/O.
          */
         goto reject;
     }