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.