tests/guest-debug: introduce basic gdbstub tests

The aim of these tests is to combine with an appropriate kernel
image (with symbol-file vmlinux) and check it behaves as it should.
Given a kernel it checks:

  - single step
  - software breakpoint
  - hardware breakpoint
  - access, read and write watchpoints

On success it returns 0 to the calling process.

I've not plumbed this into the "make check" logic though as we need a
solution for providing non-host binaries to the tests. However the test
is structured to work with pretty much any Linux kernel image as it
uses the basic kernel_init code which is common across architectures.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1449599553-24713-7-git-send-email-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
