Make the error reporting back to xm just a little bit less insane, and fix a

Make the error reporting back to xm just a little bit less insane, and fix a
missing import inside blkif.  Fix the command parsing for op_device_destroy and
op_device_configure -- the device IDs should be strings, to allow the user to
specify devices by name as well as number.

This is the second half to the fixes for bug #315.

Signed-off-by: Ewan Mellor <ewan@xensource.com>