KVM: SEV: unify cgroup cleanup code for svm_vm_migrate_from

Use the same cleanup code independent of whether the cgroup to be
uncharged and unref'd is the source or the destination cgroup.  Use a
bool to track whether the destination cgroup has been charged, which also
fixes a bug in the error case: the destination cgroup must be uncharged
only if it does not match the source.

Fixes: b5663931

 ("KVM: SEV: Add support for SEV intra host migration")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>