acpi: Move setters/getters of oem fields to X86MachineState

acpi: Move setters/getters of oem fields to X86MachineState

The code that sets/gets oem fields is duplicated in both PC and MICROVM
variants. This commit moves it to X86MachineState so that all x86
variants can use it and duplication is removed.

Signed-off-by: Marian Postevca <posteuca@mutex.one>
Message-Id: <20210221001737.24499-2-posteuca@mutex.one>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>