RISC-V: KVM: use vma_lookup() instead of find_vma_intersection()

vma_lookup() finds the vma of a specific address with a cleaner
interface and is more readable.

Signed-off-by: Bo Liu <liubo03@inspur.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>