rust: Kconfig: Factor out whether HPET is Rust or C

rust: Kconfig: Factor out whether HPET is Rust or C

Currently we require everywhere that wants to know if there
is an HPET device to check for "CONFIG_HPET || CONFIG_X_HPET_RUST".
Factor out whether the HPET device is Rust or C into a separate
Kconfig stanza, so that CONFIG_HPET means "there is an HPET",
and whether this has pulled in CONFIG_X_HPET_RUST or CONFIG_HPET_C
is something the rest of QEMU can ignore.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Link: https://lore.kernel.org/r/20250319193110.1565578-3-peter.maydell@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>