arm64: timer: Fix vtimer interrupt test

The timer irq_handler is supposed to mask the timer signal, but unfortunately
also disables the timer at the same time, even though we loop and wait on
ISTATUS to become set.

According to the ARM ARM, "When the value of the ENABLE bit is 0, the
ISTATUS field is UNKNOWN."  This test happens to work on AMD Seattle, but
doesn't work on Mustang or on QEMU with TCG.

Fix the problem by preserving the enable bit in the irq handler.

Signed-off-by: Christoffer Dall <cdall@linaro.org>
Message-Id: <20170713192009.10069-2-cdall@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>