Don't exit() in config_error()

Propagating errors up the call chain is tedious.  In startup code, we
can take a shortcut: terminate the program.  This is wrong elsewhere,
the monitor in particular.

config_error() tries to cater for both customers: it terminates the
program unless its mon parameter tells it it's working for the
monitor.

Its users need to return status anyway (unless passing a null mon
argument, which none do), which their users need to check.  So this
automatic exit buys us exactly nothing useful.  Only the dangerous
delusion that we can get away without returning status.  Some of its
users fell for that.  Their callers continue executing after failure
when working for the monitor.

This bites monitor command host_net_add in two places:

* net_slirp_init() continues after slirp_hostfwd(), slirp_guestfwd(),
  or slirp_smb() failed, and may end up reporting success.  This
  happens for "host_net_add user guestfwd=foo": it complains about the
  invalid guest forwarding rule, then happily creates the user network
  without guest forwarding.

* net_client_init() can't detect slirp_guestfwd() failure, and gets
  fooled by net_slirp_init() lying about success.  Suppresses its
  "Could not initialize device" message.

Add the missing error reporting, make sure errors are checked, and
drop the exit() from 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>
