KVM: arm64: Don't write to Rt unless sys_reg emulation succeeds

emulate_sys_reg() returns 1 unconditionally, even though a a system
register access can fail. Furthermore, kvm_handle_sys_reg() writes to Rt
for every register read, regardless of if it actually succeeded.

Though this pattern is safe (as params.regval is initialized with the
current value of Rt) it is a bit ugly. Indicate failure if the register
access could not be emulated and only write to Rt on success.

Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220503060205.2823727-3-oupton@google.com