conf: add support for setting OEM strings SMBIOS data fields

conf: add support for setting OEM strings SMBIOS data fields

The OEM strings table in SMBIOS allows the vendor to pass arbitrary
strings into the guest OS. This can be used as a way to pass data to an
application like cloud-init, or potentially as an alternative to the
kernel command line for OS installers where you can't modify the install
ISO image to change the kernel args.

As an example, consider if cloud-init and anaconda supported OEM strings
you could use something like

    <oemStrings>
      <entry>cloud-init:ds=nocloud-net;s=http://10.10.0.1:8000/</entry>
      <entry>anaconda:method=http://dl.fedoraproject.org/pub/fedora/linux/releases/25/x86_64/os</entry>
    </oemStrings>

use of a application specific prefix as illustrated above is
recommended, but not mandated, so that an app can reliably identify
which of the many OEM strings are targetted at it.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>