i386/fw_cfg: move hpet_cfg definition to hpet.c
HPET device needs to access and update hpet_cfg variable, but now it is
defined in hw/i386/fw_cfg.c and Rust code can't access it.
Move hpet_cfg definition to hpet.c (and rename it to hpet_fw_cfg). This
allows Rust HPET device implements its own global hpet_fw_cfg variable,
and will further reduce the use of unsafe C code access and calls in the
Rust HPET implementation.
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250210030051.2562726-2-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>