Get buffer function now raises exception trying to access device pointer

```
In [1]: import dpctl

In [2]: ms = dpctl.MemoryUSMShared(256)

In [3]: md = dpctl.MemoryUSMDevice(256)

In [4]: ms2 = dpctl.MemoryUSMShared(md)

In [5]: memoryview(ms2)[1]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-d512baf01446> in <module>()
----> 1 memoryview(ms2)[1]

/localdisk/work/opavlyk/repos/dpctl/dpctl/_memory.pyx in dpctl._memory.MemoryUSMShared.__getbuffer__()
    448
        449     def __getbuffer__(self, Py_buffer *buffer, int flags):
	--> 450         self._getbuffer(buffer, flags)
	    451
	        452

/localdisk/work/opavlyk/repos/dpctl/dpctl/_memory.pyx in dpctl._memory.Memory._getbuffer()
    250             ctx.get_context_ref())
        251         if kind == b'device':
	--> 252             raise ValueError('USM Device memory is not host accessible')
	    253         buffer.buf = <char *>self.memory_ptr
	        254         buffer.format = 'B'                     # byte

ValueError: USM Device memory is not host accessible
```

Author: oleksandr-pavlyk

dpctl/_memory.pyx

@@ -244,6 +244,12 @@ cdef class Memory:
 
     cdef _getbuffer(self, Py_buffer *buffer, int flags):
         # memory_ptr is Ref which is pointer to SYCL type. For USM it is void*.
+        cdef SyclContext ctx = self._context
+        cdef const char * kind = DPPLUSM_GetPointerType(
+            self.memory_ptr,
+            ctx.get_context_ref())
+        if kind == b'device':
+            raise ValueError('USM Device memory is not host accessible')
         buffer.buf = <char *>self.memory_ptr
         buffer.format = 'B'                     # byte
         buffer.internal = NULL                  # see References