hw/xfree86: Support ACPI without APM.

On systems with ACPI but disabled APM (e.g. --disable-linux-apm)
the code does not compile due to preprocessor directives.

If APM is disabled, the final return statement is considered to
be part of ACPI's last if-statement, leading to a function which
has no final return statement at all.

I have refactored the code so ACPI and APM are independent of each
other.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>