Add qemu_opts_validate() for post parsing validation

Several qemu command line options have a parameter whose value affects
what other parameters are accepted for the option.

In these cases, we can have an empty description table in the
QemuOptsList and once the option has been parsed we can use a suitable
description table to validate the other parameters based on the value of
that parameter.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
