x86/boot: fix early error output

In 9180f53655 a change was made to the send_chr function to take in
C-strings and output a character at a time until a NULL was encountered.
However, when there is no VGA there is no code to increment the current
character position resulting in an endless loop of the first character.
This moves the (implicit) increment such that it occurs in all cases.

Signed-off-by: David Esler <drumandstrum@gmail.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
[jb: correct title and description]
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

xen/arch/x86/boot/head.S

@@ -173,10 +173,11 @@ not_multiboot:
 .Lget_vtb:
         mov     sym_esi(vga_text_buffer),%edi
 .Lsend_chr:
-        mov     (%esi),%bl
-        test    %bl,%bl        # Terminate on '\0' sentinel
+        lodsb
+        test    %al,%al        # Terminate on '\0' sentinel
         je      .Lhalt
         mov     $0x3f8+5,%dx   # UART Line Status Register
+        mov     %al,%bl
 2:      in      %dx,%al
         test    $0x20,%al      # Test THR Empty flag
         je      2b
@@ -185,7 +186,7 @@ not_multiboot:
         out     %al,%dx        # Send a character over the serial line
         test    %edi,%edi      # Is the VGA text buffer available?
         jz      .Lsend_chr
-        movsb                  # Write a character to the VGA text buffer
+        stosb                  # Write a character to the VGA text buffer
         mov     $7,%al
         stosb                  # Write an attribute to the VGA text buffer
         jmp     .Lsend_chr