While fully converted handlers are not supposed to print anything when
running in a QMP monitor, they are free to print in a human monitor.
For instance, device_add (not yet converted) prints help, and will
continue to do so after conversion.

Moreover, utility functions converted to QError should remain usable
from unconverted handlers.

Two problems:

* handler_audit() complains when a converted handler prints.  Limit
  that to QMP monitors.

* With QMP, handlers need to pass the error object by way of
  monitor_set_error().  However, we do that both for QMP and for the
  human monitor.  The human monitor prints the error object after the
  handler returns.  If the handler prints anything else, that output
  "overtakes" the error message.

  Limit use of monitor_set_error() to QMP monitors.  Update
  handler_audit() accordingly.

New LOC_CMDLINE.  Use it for tracking option with argument in
lookup_opt().  We now report errors like this

    qemu: -device smbus-eeprom: Did not find I2C bus for smbus-eeprom

New LOC_FILE.  Use it for tracking file name and line number in
qemu_config_parse().  We now report errors like

    qemu:foo.conf:42: Did not find I2C bus for smbus-eeprom

In particular, gems like this message:

    -device: no driver specified

become almost nice now:

    qemu:foo.conf:44: -device: no driver specified

(A later commit will get rid of the bogus -device:)

New struct Location holds a location.  So far, the only location is
LOC_NONE, so this doesn't do anything useful yet.

Passing the current location all over the place would be too
cumbersome.  Hide it away in static cur_loc instead, and provide
accessors.  Print it in error_report().

Store it in QError, and print it in qerror_print().

Store it in QemuOpt, for use by qemu_opts_foreach().  This makes
error_report() do the right thing when it runs within
qemu_opts_foreach().

We may still have to store it in other data structures holding user
input for better error messages.  Left for another day.

error_report() terminates the message with a newline.  Strip it it
from its arguments.

This fixes a few error messages lacking a newline:
net_handle_fd_param()'s "No file descriptor named %s found", and
tap_open()'s "vnet_hdr=1 requested, but no kernel support for
IFF_VNET_HDR available" (all three versions).

There's one place that passes arguments without newlines
intentionally: load_vmstate().  Fix it up.

qemu_error_sink can either point to a monitor or a file.  In practice,
it always points to the current monitor if we have one, else to
stderr.  Simply route errors to the current monitor or else to stderr,
and remove qemu_error_sink along with the functions to control it.

Actually, the old code switches the sink slightly later, in
handle_user_command() and handle_qmp_command(), than it gets switched
now, implicitly, by setting the current monitor in monitor_read() and
monitor_control_read().  Likewise, it switches back slightly earlier
(same places).  Doesn't make a difference, because there are no calls
of qemu_error() in between.

They're about reporting errors, not about the monitor.