libmemif: timer causes spin at 100% cpu
libmemif: timer causes spin at 100% cpu

In case of timer is set and expires:
1. timer_fd is readable and stay readable.
2. timer_fd is part of epoll_fd set. This makes epoll_pwait()
   calls return immediately instead of sleeping.

Type: fix

Signed-off-by: Daniel Béreš <0950b222fa3efece2311d75363885ebb4493b0ca@cisco.com>
Change-Id: I9b228464fe45b83def9b182c885d6febf428049c

FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp