In most cases after vchan close program terminates (or reexec itself to wait
for next connection), so cleanup isn't needed (not sure how about evtchn and
shared pages...).
But in case of stubdom it is needed to cleanup before waiting for new
connection - we can't reexec agent there.
Use features macros (QREXEC_RING_V2 and ASYNC_INIT) instead of directly
depending on building environment. The macros are turned on (when required) in
libvchan.h based on target system.
