KVM: SVM: Add support for KVM_SEV_SEND_CANCEL command

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 <[email protected]>
Reviewed-by: Brijesh Singh <[email protected]>
Signed-off-by: Steve Rutherford <[email protected]>
Message-Id: <20210412194408.2458827[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>