Add a magic field into struct nbd_handle, and assert that this is set
in most libnbd calls.  For a small overhead this adds additional
checks against some kinds of memory corruption and clients passing
invalid pointers.

Notes:

- Cause an assert fail rather than an error return as these
  would be unexpected internal errors.

- Check happens outside the handle lock, which should be safe and
  slightly reduces the overhead in parallel cases (and is necessary
  because we can only take the handle lock after checking that the
  handle is valid).

- Use a small constant so that the check can be turned into a single
  instruction (with immediate) on x86-64:

      11:       48 81 3f bd bd 00 00    cmpq   $0xbdbd,(%rdi)

  Larger constants need 'movabs' and consume an extra register while
  being no safer, for example:

       4:       48 b8 cb cb cb cb cb    movabs $0xbdbdbcbcbcbcbcb,%rax
      1b:       48 39 07                cmp    %rax,(%rdi)