Fix a crash in gdbserver-xen where it references unmapped memory if

Fix a crash in gdbserver-xen where it references unmapped memory if
the page directory / page table pages are not present (for example
through dereferencing NULL in the gdb session). Changes:

- map_domain_va_32 has been rewritten to match map_domain_va_pae and
  map_domain_va_64.

- All three functions have been extended to handle multiple vCPUs,
  (untested, but shuld work), and check for _PAGE_PRESENT.

- Keir's workaround for the compile error in map_domain_va_64 has been
  removed and hopefully fixed.

Signed-Off-By: Simon Kagstrom <simon.kagstrom@bth.se>