Drop config_error(), use qemu_error() instead

Drop config_error(), use qemu_error() instead

Diagnostic output goes to stderr, except when we're in a monitor
command, when it goes to the monitor instead.

config_error() implements this with a monitor argument: if it's
non-null, report there, else to stderr.  This obliges us to pass the
monitor down various call chains, to make it available to
config_error().

The recently created qemu_error() doesn't need a monitor argument to
route output.  Use it.

There's one user-visible difference: config_error() prepended "qemu: "
to a message bound for stderr.  qemu_error() doesn't, which means the
prefix goes away with this commit.  If such a prefix is desired for
stderr, then I figure it should be slapped on all error messages, not
just the ones that used to go through config_error().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>