vmm: fix test_epoll_stdin_event
vmm: fix test_epoll_stdin_event

This unit tests makes sure that calling
epoll_ctl_add on STDIN_FILENO succeeds.
However, if the unit tests are run through
ssh without a pseudo terminal, adding STDIN_FILENO
would return EPERM. Adjust unit test to not panic
in case of EPERM.

Signed-off-by: Diana Popa <4240422768c1b315e51818a074064501037757cd@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker