accel/tcg: Relax va restrictions on 64-bit guests

accel/tcg: Relax va restrictions on 64-bit guests

We cannot at present limit a 64-bit guest to a virtual address
space smaller than the host.  It will mostly work to ignore this
limitation, except if the guest uses high bits of the address
space for tags.  But it will certainly work better, as presently
we can wind up failing to allocate the guest stack.

Widen our user-only page tree to the host or abi pointer width.
Remove the workaround for this problem from target/alpha.
Always validate guest addresses vs reserved_va, as there we
control allocation ourselves.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200513175134.19619-7-alex.bennee@linaro.org>