bus/vmbus: add missing barrier
bus/vmbus: add missing barrier

The check for event ring being empty needs a barrier
to avoid any over aggressive optimization.
This is same barrier as Linux kernel.

Signed-off-by: Stephen Hemminger <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@networkplumber.org>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk