target/arm: fix AArch64 virtual address space size

Since QEMU does not support the ARMv8.2-LVA, Large Virtual Address,
extension (yet), the VA address space is 48-bits plus a sign bit. User
mode can only handle the positive half of the address space, so that
makes a limit of 48 bits.

(With LVA, it would be 53 and 52 bits respectively.)

The incorrectly large address space conflicts with PAuth instructions,
which use bits 48-54 and 56-63 for the pointer authentication code. This
also conflicts with (as yet unsupported by QEMU) data tagging and with
the ARMv8.5-MTE extension.

Signed-off-by: Remi Denis-Courmont <remi.denis.courmont@huawei.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>