iommufd/tsm: Add support for tsm_map_ioctl

Introduce tsm_map_ioctl to enable the VMM to allocate private guest
memory during the transition from shared to private memory. This ensures
guest RAM is fully allocated up front, preventing page faults that can
interfere with passthrough device functionality.

Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>