swtpm: cuse: Restrict opening CUSE device to one openable file descriptor

swtpm: cuse: Restrict opening CUSE device to one openable file descriptor

Restrict the opening of the CUSE device to one single file descriptor. We
can modify the CUSE TPM in this way since the kernel's /dev/tpm0 cannot be
opened multiple times, either, and the CUSE TPM should behave in the same
way.

Adjust test the partial reads case to only open CUSE device file once by
using a python program. Close the open file descriptor 100 before using
swtpm_ioctl to avoid failures.

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