rust: hpet: decode HPET registers into enums

rust: hpet: decode HPET registers into enums

Generalize timer_and_addr() to decode all registers into a single enum
HPETRegister, and use the TryInto derive to separate valid and
invalid values.

The main advantage lies in checking that all registers are enumerated
in the "match" statements.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>