smbios: Convert to QemuOpts

smbios: Convert to QemuOpts

So that it can be set in config file for -readconfig.

This tightens parsing of -smbios, and makes it more consistent with
other options: unknown parameters are rejected, numbers with trailing
junk are rejected, when a parameter is given multiple times, last
rather than first wins, ...

MST: drop one chunk to fix build errors

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>