python: Accept buffers in nbd.Buffer.from_bytearray()

Prior to this patch, the following fails, but at least seems to give a
sensible error:

$ nbdsh -c 'nbd.Buffer.from_bytearray(b"1"*8)'
Traceback (most recent call last):
  File "/usr/lib64/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib64/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/usr/lib64/python3.10/site-packages/nbd.py", line 2726, in <module>
    nbdsh.shell()
  File "/usr/lib64/python3.10/site-packages/nbdsh.py", line 139, in shell
    exec(c, d, d)
  File "<string>", line 1, in <module>
  File "/usr/lib64/python3.10/site-packages/nbd.py", line 132, in from_bytearray
    o = libnbdmod.aio_buffer_from_bytearray(ba)
RuntimeError: parameter is not a bytearray

while this version 1 byte longer flat out segfaults:

$ nbdsh -c 'nbd.Buffer.from_bytearray(b"1"*9)'

and this one is subtly different:

$ nbdsh -c 'nbd.Buffer.from_bytearray(h)'
Traceback (most recent call last):
...
  File "/usr/lib64/python3.10/site-packages/nbd.py", line 132, in from_bytearray
    o = libnbdmod.aio_buffer_from_bytearray(ba)
MemoryError

That's because PyByteArray_AsString() blindly assumes that its
argument is a PyByteArray, and goes haywire when it is not, unless we
got lucky that the incorrectly-typed object behaves similarly enough
(which, for byte literals, is size-dependent).

But Python already has a handy way to convert any object that supports
the buffer interface into a bytearray.  Using it, we can now support
many more parameters; passing in b"1"*9 now correctly creates a 9-byte
buffer rather than failing.  And the error message for a non-buffer
also improves:

$ ./run nbdsh -c 'nbd.Buffer.from_bytearray(h)'
Traceback (most recent call last):
...
  File "/home/eblake/libnbd/python/nbd.py", line 132, in from_bytearray
    o = libnbdmod.aio_buffer_from_bytearray(ba)
TypeError: cannot convert 'NBD' object to bytearray

Now we get a reliable TypeError, which is always better than an
unexpected MemoryError or segfault.

Also update unit test 580 to cover more buffer functions.

It may still be worth a future API change to use PyMemoryView for less
copying in and out of C structures and thus more performance, but
that's a bigger task.

Reviewed-by: Richard W.M. Jones <rjones@redhat.com>