acpi: add function to extract oem_id and oem_table_id from the user's SLIC

The acpi_get_slic_oem() function stores pointers to these fields in the
(first) SLIC table that the user passes in with the -acpitable switch.

Cc: "Michael S. Tsirkin" <mst@redhat.com> (supporter:ACPI/SMBIOS)
Cc: Igor Mammedov <imammedo@redhat.com> (supporter:ACPI/SMBIOS)
Cc: Richard W.M. Jones <rjones@redhat.com>
Cc: Aleksei Kovura <alex3kov@zoho.com>
Cc: Michael Tokarev <mjt@tls.msk.ru>
Cc: Steven Newbury <steve@snewbury.org.uk>
RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1248758
LP: https://bugs.launchpad.net/qemu/+bug/1533848
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Steven Newbury <steve@snewbury.org.uk>
