qemu: Support per-process memory accounting for iommufd

qemu: Support per-process memory accounting for iommufd

Implement the IOMMU_OPTION_RLIMIT_MODE
ioctl to set per-process memory accounting for
iommufd. This prevents ENOMEM errors from the
default per-user memory accounting when multiple
VMs under the libvirt-qemu user have their pinned
memory summed and checked against a per-process
RLIMIT_MEMLOCK limit.

Signed-off-by: Nathan Chen <nathanc@nvidia.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>