KVM: arm64: Fully zero the vcpu state on reset

On vcpu reset, we expect all the registers to be brought back
to their initial state, which happens to be a bunch of zeroes.

However, some recent commit broke this, and is now leaving a bunch
of registers (such as the FP state) with whatever was left by the
guest. My bad.

Zero the reset of the state (32bit SPSRs and FPSIMD state).

Cc: stable@vger.kernel.org
Fixes: e47c2055 ("KVM: arm64: Make struct kvm_regs userspace-only")
Signed-off-by: Marc Zyngier <maz@kernel.org>