target/riscv: Clear CSR values at reset and sync MPSTATE with host

target/riscv: Clear CSR values at reset and sync MPSTATE with host

This patch fixes guest reboot errors when using KVM.

There are two issues when rebooting a guest using KVM
1. When the guest initiates a reboot the host is unable to stop the vcpu
2. When running a SMP guest the qemu monitor system_reset causes a vcpu crash

This can be fixed by clearing the CSR values at reset and syncing the
MPSTATE with the host.

Signed-off-by: liguang.zhang <liguang.zhang@hexintek.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20230913091332.17355-1-18622748025@163.com>
[ Changes by AF
 - Fixup commit message
 - Fixup patch style
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>