block.c: Fix type of IoOperationType variable in send_qmp_error_event()

The local variable 'ac' in send_qmp_error_event() is declared with the
wrong type, which causes clang to complain when it is initialized
and again when it is used:

block.c:3655:20: warning: implicit conversion from enumeration type 'enum IoOperationType' to different enumeration type 'BlockErrorAction' (aka 'enum BlockErrorAction') [-Wenum-conversion]
    ac = is_read ? IO_OPERATION_TYPE_READ : IO_OPERATION_TYPE_WRITE;
       ~           ^~~~~~~~~~~~~~~~~~~~~~
block.c:3655:45: warning: implicit conversion from enumeration type 'enum IoOperationType' to different enumeration type 'BlockErrorAction' (aka 'enum BlockErrorAction') [-Wenum-conversion]
    ac = is_read ? IO_OPERATION_TYPE_READ : IO_OPERATION_TYPE_WRITE;
       ~                                    ^~~~~~~~~~~~~~~~~~~~~~~
block.c:3656:62: warning: implicit conversion from enumeration type 'BlockErrorAction' (aka 'enum BlockErrorAction') to different enumeration type 'IoOperationType' (aka 'enum IoOperationType') [-Wenum-conversion]
    qapi_event_send_block_io_error(bdrv_get_device_name(bs), ac, action,
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                           ^~

Correct the type to IoOperationType, and rename the variable
to 'optype' to match its correct type.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Message-id: 1412969583-21045-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
