qemu_validate: Check whether granule of virtio-iommu is supported

qemu_validate: Check whether granule of virtio-iommu is supported

Just like with other features, check whether QEMU supports them
based on capabilities. Now, instead of inventing a new QEMU
capability, an existing one can be used:
QEMU_CAPS_VIRTIO_IOMMU_AW_BITS.

This is because the aw-bits and granule attributes were
introduced into QEMU in close succession (v9.0.0-rc0~9^2~7
v9.0.0-rc0~9^2~11), neither can be disabled at compile time and
backporting just one without the other makes almost no sense.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>