migration: Use error_reportf_err() instead of monitor_printf()

migration: Use error_reportf_err() instead of monitor_printf()

Both error_reportf_err() and monitor_printf() print to the same
destination when monitor_printf() is used correctly, i.e. within an
HMP monitor.  Elsewhere, monitor_printf() does nothing, while
error_reportf_err() reports to stderr.

Both changed functions are HMP command handlers.  These should only
run within an HMP monitor.

Unlike monitor_printf(), error_reportf_err() uses the error whole
instead of just its message obtained with error_get_pretty().  This
avoids suppressing its hint (see commit 50b7b00), but I don't think
the errors touched in this commit can come with hints.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-15-git-send-email-armbru@redhat.com>