Fix gdb debugging of hypervisor.

Fix gdb debugging of hypervisor.

This patch:
  * enables the gdbstubs to properly access hypervisor memory;
  * prevents an assertion failure in __spurious_page_fault's call
    to map_domain_page if such accesses fail, by testing in_irq();
  * prints some additional helpful messages;
  * fixes the endianness of register transfers from the gdbstubs
    so that gdb is much less confused.
  * fixes the documentation in docs/misc/crashdb.txt

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>