xenctx: Clean up stack trace when hypercall_page not in symbol table

xenctx: Clean up stack trace when hypercall_page not in symbol table

Before:

Call Trace:
  [<ffffffff8006b2b0>] default_idle+0x29  <--
  [<ffffffff80048d19>] cpu_idle+0x95
  [<ffffffff803e7801>] start_kernel+0x220
  [<0000000000000000>] startup_64+0x80000000
  [<ffffffff803e722f>] x86_64_start_kernel+0x22f
  [<0000000000000000>] startup_64+0x80000000
  [<0000000000000000>] startup_64+0x80000000
  [<0000000000000000>] startup_64+0x80000000
  [<0000000000000000>] startup_64+0x80000000

After:

Call Trace:
  [<ffffffff8006b2b0>] default_idle+0x29  <--
  [<ffffffff80048d19>] cpu_idle+0x95
  [<ffffffff803e7801>] start_kernel+0x220
  [<ffffffff803e722f>] x86_64_start_kernel+0x22f

Signed-off-by: Don Slutz <Don@CloudSwitch.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>