arm: Handle exits from undecoded load/store instructions

KVM occasionally encounters guests that attempt to access memory outside
the registered RAM memory slots using instructions that don't provide
decoding information in the ESR_EL2 (the ISV bit is not set), and
historically this has led to the kernel printing a confusing error
message in dmesg and returning -ENOYSYS from KVM_RUN.

KVM/Arm now has KVM_CAP_ARM_NISV_TO_USER, which can be enabled from
userspace, and which allows us to handle this with a little bit more
helpful information to the user.  For example, we can at least tell the
user if the guest just hit a hole in the guest's memory map, or if this
appeared to be an attempt at doing MMIO.

Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>