tpm2: change tpm2_unseal() to accept Tpm2Context instead of device string

tpm2: change tpm2_unseal() to accept Tpm2Context instead of device string

This matches the change to tpm2_seal(), which now accepts a Tpm2Context instead
of a device string.

This also allows using the same TPM context for sealing and unsealing, which
will be required by (future) test code when sealing/unsealing using a transient
key.