accel/tcg/cputlb: avoid recursive BQL (fixes #1706296)

accel/tcg/cputlb: avoid recursive BQL (fixes #1706296)

The mmio path (see exec.c:prepare_mmio_access) already protects itself
against recursive locking and it makes sense to do the same for
io_readx/writex. Otherwise any helper running in the BQL context will
assert when it attempts to write to device memory as in the case of
the bug report.

Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
CC: Richard Jones <[email protected]>
CC: Paolo Bonzini <[email protected]>
CC: [email protected]
Message-Id: <20170921110625[email protected]>
Signed-off-by: Richard Henderson <[email protected]>