os-support/solaris: Set IOPL for input thread too

Since the Solaris kernel tracks IOPL per thread, and doesn't inherit
raised IOPL levels when creating a new thread, we need to turn it on
in the input thread for input drivers like vmmouse that need register
access to work correctly.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
(cherry picked from commit 12769c51)