ANDROID: KVM: arm64: Keep the pKVM private range under 1GiB

The hypervisor memory pool is sized to allow mapping up to 1GiB of data
in the 'private' range of the hypervisor. However, this is currently
not enforced in any way, which might become a problem as private range
mappings are used more and more (e.g. from pKVM modules).

Enforce the 1GiB limit at allocation time, and while at it, rename
__io_map_base to __private_range_base for consistency.

Bug: 244543039
Change-Id: I32c9145ba331309b49428ff461a41c94ea0c1512
Signed-off-by: Quentin Perret <qperret@google.com>