LoongArch: KVM: Set host with kernel mode when switch to VM mode

PRMD register is only meaningful on the beginning stage of exception
entry, and it is overwritten with nested irq or exception.

When CPU runs in VM mode, interrupt need be enabled on host. And the
mode for host had better be kernel mode rather than random or user mode.

When VM is running, the running mode with top command comes from CRMD
register, and running mode should be kernel mode since kernel function
is executing with perf command. It needs be consistent with both top and
perf command.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>