ocaml/xenctrl: Fix stub_xc_readconsolering()

ocaml/xenctrl: Fix stub_xc_readconsolering()

The Ocaml stub to retrieve the hypervisor console ring had a few problems.

 * A single 32k buffer would truncate a large console ring.
 * The buffer was static and not under the protection of the Ocaml GC lock so
   could be clobbered by concurrent accesses.
 * Embedded NUL characters would cause caml_copy_string() (which is strlen()
   based) to truncate the buffer.

The function is rewritten from scratch, using the same algorithm as the python
stubs, but uses the protection of the Ocaml GC lock to maintain a static
running total of the ring size, to avoid redundant realloc()ing in future
calls.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Dave Scott <dave.scott@eu.citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: David Scott <dave.scott@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>