vl: Clean up parsing of -boot option argument

Commit 3d3b8303 threw in some QemuOpts parsing without replacing the
existing ad hoc parser, resulting in a confusing mess.  Clean it up.

Two user-visible changes:

1. Invalid options are reported more nicely.  Before:

        qemu: unknown boot parameter 'x' in 'x=y'

   After:

        qemu-system-x86_64: -boot x=y: Invalid parameter 'x'

2. If -boot is given multiple times, options accumulate, just like for
   -machine.  Before, only options order, once and menu accumulated.
   For the other ones, all but the first -boot in non-legacy syntax
   got simply ignored.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Message-id: 1371208516-7857-2-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>