qemu: Fix error when migration with shared TPM storage is unsupported

qemu: Fix error when migration with shared TPM storage is unsupported

The VIR_ERR_NO_SUPPORT error is supposed to be used for unsupported
driver APIs. It is incorrectly used when swtpm does not support
migration with shared storage resulting in a rather strange error
message:

    this function is not supported by the connection driver: the running
    swtpm does not support migration with shared storage

The correct VIR_ERR_OPERATION_UNSUPPORTED error code provides a much
better message:

    Operation not supported: the running swtpm does not support
    migration with shared storage

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>