glib: Add compat wrapper for g_poll on old glib

Older glib doesn't implement g_poll(). Most notably the glib version in use
on SLE11 is on 2.18 which is hit by this.

We do want to use g_poll() in the source however. So on older systems, just
wrap it with functions that do exist on older versions.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>