KVM: SEV: Prohibit migration of a VM that has mirrors

VMs that mirror an encryption context rely on the owner to keep the
ASID allocated.  Performing a KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM
would cause a dangling ASID:

1. copy context from A to B (gets ref to A)
2. move context from A to L (moves ASID from A to L)
3. close L (releases ASID from L, B still references it)

The right way to do the handoff instead is to create a fresh mirror VM
on the destination first:

1. copy context from A to B (gets ref to A)
[later] 2. close B (releases ref to A)
3. move context from A to L (moves ASID from A to L)
4. copy context from L to M

So, catch the situation by adding a count of how many VMs are
mirroring this one's encryption context.

Fixes: 0b020f5a ("KVM: SEV: Add support for SEV-ES intra host migration")
Message-Id: <20211123005036.2954379-11-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>