virterror: Add error message for unsupported operations.

virterror: Add error message for unsupported operations.

This patch introduces a new error code VIR_ERR_OPERATION_UNSUPPORTED to
mark error messages regarding operations that failed due to lack of
support on the hypervisor or other than libvirt issues.

The code is first used in reporting error if qemu does not support block
IO tuning variables yielding error message:
error: Unable to get block I/O throttle parameters
error: Operation not supported: block_io_throttle field
'total_bytes_sec' missing in qemu's output

instead of:
error: Unable to get block I/O throttle parameters
error: internal error cannot read total_bytes_sec