hw/riscv/virt.c, riscv-iommu-sys.c: add MSIx support

hw/riscv/virt.c, riscv-iommu-sys.c: add MSIx support

MSIx support is added in the RISC-V IOMMU platform device by including
the required MSIx facilities to alow software to properly setup the MSIx
subsystem.

We took inspiration of what is being done in the riscv-iommu-pci device,
mainly msix_init() and msix_notify(), while keeping in mind that
riscv-iommu-sys isn't a true PCI device and we don't need to copy/paste
all the contents of these MSIx functions.

Two extra MSI MemoryRegions were added: 'msix-table' and 'msix-pba'.
They are used to manage r/w of the MSI table and Pending Bit Array (PBA)
respectively. Both are subregions of the main IOMMU memory region,
iommu->regs_mr, initialized during riscv_iommu_realize(), and each one
has their own handlers for MSIx reads and writes.

This is the expected memory map when using this device in the 'virt'
machine:

    0000000003010000-0000000003010fff (prio 0, i/o): riscv-iommu-regs
      0000000003010300-000000000301034f (prio 0, i/o): msix-table
      0000000003010400-0000000003010407 (prio 0, i/o): msix-pba

We're now able to set IGS to RISCV_IOMMU_CAP_IGS_BOTH, and userspace is
free to decide which interrupt model to use.

Enabling MSIx support for this device in the 'virt' machine requires
adding 'msi-parent' in the iommu-sys DT.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241106133407.604587-6-dbarboza@ventanamicro.com>
[ Changes by AF:
 - Used PRIx64 in trace
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>