Fix error message and wait time for xm block-detach command.

Fix error message and wait time for xm block-detach command.

 - Wait time
    When xm requests a block device detach to xend, xm makes two
    requests. At first, xm requests the block device detach by device
    class 'vbd'. Next, xm requests the block device detaching by
    device class 'tap'.
    As a result, the wait time is 200 seconds because each of
    the block device detaching requests causes time-out.
 - Misleading error message
    Because the last request is by device class 'tap' to xend,
    the keyword "(tap)" is included in the error message.

This patch fixes the number of times of the block device detaching
request to one time.  At first, xm makes inquiries about device
class of a detaching target device to xend.  Then xm requires the
block device detaching by xend returned device class.  The wait
time becomes 100 seconds because the block device detaching request
is one time.  And the error message is also fixed.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>