acpi: build_append_nameseg(): add padding if necessary

acpi: build_append_nameseg(): add padding if necessary

According to ACPI spec NameSeg shorter than 4 characters
must be padded up to 4 characters with "_" symbol.
ACPI 5.0:  20.2.2 "Name Objects Encoding"

Do it in build_append_nameseg() so that caller shouldn't know
or care about it.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>