docs: update riscv/virt.rst with kernel-irqchip=split support

Also add a new page, docs/specs/riscv-aia.rst, where we're documenting
the state of AIA support in QEMU w.r.t the controllers being emulated or
not depending on the AIA and accelerator settings.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241119191706.718860-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/system/riscv/virt.rst

@@ -129,6 +129,13 @@ The following machine-specific options are supported:
   MSIs. When not specified, this option is assumed to be "none" which selects
   SiFive PLIC to handle wired interrupts.
 
+  This option also interacts with '-accel kvm'.  When using "aia=aplic-imsic"
+  with KVM, it is possible to set the use of the kernel irqchip in split mode
+  by using "-accel kvm,kernel-irqchip=split".  In this case the ``virt`` machine
+  will emulate the APLIC controller instead of using the APLIC controller from
+  the irqchip.  See :ref:`riscv-aia` for more details on all available AIA
+  modes.
+
 - aia-guests=nnn
 
   The number of per-HART VS-level AIA IMSIC pages to be emulated for a guest