smbios: support setting OEM strings table

The cloud-init program currently allows fetching of its data by repurposing of
the 'system' type 'serial' field. This is a clear abuse of the serial field that
would clash with other valid usage a virt management app might have for that
field.

Fortunately the SMBIOS defines an "OEM Strings" table whose puporse is to allow
exposing of arbitrary vendor specific strings to the operating system. This is
perfect for use with cloud-init, or as a way to pass arguments to OS installers
such as anaconda.

This patch makes it easier to support this with QEMU. e.g.

  $QEMU -smbios type=11,value=Hello,value=World,value=Tricky,,value=test

Which results in the guest seeing dmidecode data

  Handle 0x0E00, DMI type 11, 5 bytes
  OEM Strings
          String 1: Hello
          String 2: World
          String 3: Tricky,value=test

It is suggested that any app wanting to make use of this OEM strings capability
for accepting data from the host mgmt layer should use its name as a string
prefix. e.g. to expose OEM strings targetting both cloud init and anaconda in
parallel the mgmt app could set

  $QEMU -smbios type=11,value=cloud-init:ds=nocloud-net;s=http://10.10.0.1:8000/,\
        value=anaconda:method=http://dl.fedoraproject.org/pub/fedora/linux/releases/25/x86_64/os

which would appear as

  Handle 0x0E00, DMI type 11, 5 bytes
  OEM Strings
          String 1: cloud-init:ds=nocloud-net;s=http://10.10.0.1:8000/
          String 2: anaconda:method=http://dl.fedoraproject.org/pub/fedora/linux/releases/25/x86_64/os

Use of such string prefixes means the app won't have to care which string slot
its data appears in.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
