swtpm: Implemented support for PTM_SET_BUFFERSIZE command

swtpm: Implemented support for PTM_SET_BUFFERSIZE command

Implement a command for setting and querying the buffer size the
TPM implementation (libtpms) is using. The setting of the
buffersize allows to reduce the size of the buffer to a size
that the interface can support so that these two sizes match
and the TPM will not produce larger responses than what the
interface can support.

Extend swtpm_ioctl with an option to set the buffersize.

Adapt the existing tests to reflect the newly supported command.
Implement a new test for getting/setting of the buffer size.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>