KVM: X86: Add per-vm stat for max rmap list size

Add a new statistic max_mmu_rmap_size, which stores the maximum size of rmap
for the vm.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210625153214.43106-2-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>