RISC-V: KVM: Save trap CSRs in kvm_riscv_vcpu_enter_exit()

We should save trap CSRs in kvm_riscv_vcpu_enter_exit() instead of
kvm_arch_vcpu_ioctl_run() so that we can do more optimized access
to HTVAL and HTINST CSRs while running under some other hypervisor.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>