KVM: SVM: Add support for KVM_SEV_SEND_CANCEL command

After completion of SEND_START, but before SEND_FINISH, the source VMM can
issue the SEND_CANCEL command to stop a migration. This is necessary so
that a cancelled migration can restart with a new target later.

Reviewed-by: Nathan Tempelman <natet@google.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Steve Rutherford <srutherford@google.com>
Message-Id: <20210412194408.2458827-1-srutherford@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>