KVM: arm64: Handle deferred SErrors consumed on guest exit

On systems with VHE, the RAS extensions and IESB support, KVM gets an
implicit ESB whenever it enters/exits a guest, because the host sets
SCTLR_EL1.IESB.

To prevent errors being lost, add code to __guest_exit() to read DISR_EL1,
and save it in the kvm_vcpu_fault_info. Add code to handle_exit() to
process this deferred SError. This data is in addition to the reason the
guest exitted.

Future patches may add a firmware-first callout from
kvm_handle_deferred_serror() to decode CPER records populated by firmware,
or call some arm64 arch code to process the RAS 'ERR' registers for
kernel-first handling. Without either of these, we just make a judgement
on the severity: corrected and restartable errors are ignored, all others
result it an SError being given to the guest.

On systems with EL2 but where VHE has been disabled in the build config,
add an explicit ESB in the __guest_exit() path. This lets us skip the
SError VAXorcism on all systems with the RAS extensions.

Signed-off-by: James Morse <james.morse@arm.com>