target/arm: Fix ISR_EL1 tracking when executing at EL2

The ARMv8 ARM states when executing at EL2, EL3 or Secure EL1,
ISR_EL1 shows the pending status of the physical IRQ, FIQ, or
SError interrupts.

Unfortunately, QEMU's implementation only considers the HCR_EL2
bits, and ignores the current exception level. This means a hypervisor
trying to look at its own interrupt state actually sees the guest
state, which is unexpected and breaks KVM as of Linux 5.3.

Instead, check for the running EL and return the physical bits
if not running in a virtualized context.

Fixes: 636540e9
Cc: qemu-stable@nongnu.org
Reported-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Message-id: 20191122135833.28953-1-maz@kernel.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>