vl.c: Fix regression in machine error message

vl.c: Fix regression in machine error message

Commit e1ce0c3cb (vl.c: fix regression when reading machine type
from config file) fixed the error message when the machine type
was supplied inside the config file. However now the option name
is not displayed correctly if the error happens when the machine
is specified at command line.

Running
    ./x86_64-softmmu/qemu-system-x86_64 -M q35-1.5 -redir tcp:8022::22
will result in the error message:
    qemu-system-x86_64: -redir tcp:8022::22: unsupported machine type
    Use -machine help to list supported machines

Fixed it by restoring the error location and also extracted the code
dealing with machine options into a separate function.

Reported-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1455303747-19776-2-git-send-email-ehabkost@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>