x86/xen: fix booting 32-bit pv guest

Commit 2f62f36e ("x86/xen: Make the boot CPU idle task reliable")
introduced a regression for booting 32 bit Xen PV guests: the address
of the initial stack needs to be a virtual one.

Fixes: 2f62f36e ("x86/xen: Make the boot CPU idle task reliable")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20200409070001.16675-1-jgross@suse.comSigned-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/xen-head.S

@@ -38,7 +38,7 @@ SYM_CODE_START(startup_xen)
 #ifdef CONFIG_X86_64
 	mov initial_stack(%rip), %rsp
 #else
-	mov pa(initial_stack), %esp
+	mov initial_stack, %esp
 #endif
 
 #ifdef CONFIG_X86_64