Move socket-related Solaris libraries to $solarisnetlibs and use them
for both $LIBS and $libs_qga.

Fixes build on illumos without --disable-guest-agent.
Signed-off-by: NLee Essen <lee.essen@nowonline.co.uk>
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>