ref(riscv): refactor vimsic interrupt injection

[josecm]

This PR refactors APLIC MSI injection in IMSIC guest interrupt files and fixes two issues:

1. The previous implementation assumed that MSI injection into a guest interrupt file always targeted the local vCPU. If the target vCPU was remote, it sent a CPU message to the target core so the MSI could be injected locally. However, the IMSIC MMIO interface allows a CPU to inject an MSI directly into a remote (i.e., different core) guest IMSIC via its MMIO interface. Supporting this requires:

   • Mapping guest interrupt files in the hypervisor.
   • Introducing a function that injects an MSI by writing to the appropriate guest interrupt file MMIO interface based on the target CPU and its assigned guest interrupt file.

   Alternatively, local vCPU injection could still use the AIA CSRs (as before). To simplify the code, this optimization is intentionally avoided for now. Future work may evaluate whether MMIO-based injection is sufficiently costly to justify reintroducing this optimization.

2. On updates to APLIC registers, the previous implementation iterated over all APLIC interrupts to determine whether an MSI should be forwarded to a vCPU. In most cases (e.g., enabling a single interrupt), only one interrupt needs to be considered. This PR addresses the issue by adding a third argument to vaplic_update_hart, indicating whether the call was triggered by a single interrupt state update. If the interrupt ID is invalid (e.g., when triggered by a global vAPLIC control update), the logic falls back to scanning all interrupts for forwarding.