KVM: arm64: Trap access to pVM restricted features

Trap accesses to restricted features for VMs running in protected
mode.

Access to feature registers are emulated, and only supported
features are exposed to protected VMs.

Accesses to restricted registers as well as restricted
instructions are trapped, and an undefined exception is injected
into the protected guests, i.e., with EC = 0x0 (unknown reason).
This EC is the one used, according to the Arm Architecture
Reference Manual, for unallocated or undefined system registers
or instructions.

Only affects the functionality of protected VMs. Otherwise,
should not affect non-protected VMs when KVM is running in
protected mode.

Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211010145636.1950948-11-tabba@google.com