rust/timer/hpet: define hpet_fw_cfg

rust/timer/hpet: define hpet_fw_cfg

Define HPETFwEntry structure with the same memory layout as
hpet_fw_entry in C.

Further, define the global hpet_cfg variable in Rust which is the
same as the C version. This hpet_cfg variable in Rust will replace
the C version one and allows both Rust code and C code to access it.

The Rust version of hpet_cfg is self-contained, avoiding unsafe
access to C code.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250210030051.2562726-8-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>