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

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" <[email protected]> (supporter:ACPI/SMBIOS)
Cc: Igor Mammedov <[email protected]> (supporter:ACPI/SMBIOS)
Cc: Richard W.M. Jones <[email protected]>
Cc: Aleksei Kovura <[email protected]>
Cc: Michael Tokarev <[email protected]>
Cc: Steven Newbury <[email protected]>
RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1248758
LP: https://bugs.launchpad.net/qemu/+bug/1533848
Signed-off-by: Laszlo Ersek <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Steven Newbury <[email protected]>