When shutting down SOS VM, the shared sbuf is released from guest OS, but
the per cpu sbuf pointers in hypervisor keep inact. This creates a problem
that after SOS is re-launched, hypervisor could write to the shared
buffer that no longer exists.
This patch implements sbuf_reset() and call it from reset_vcpu() to
reset sbuf pointers.
Tracked-On: #2700
Signed-off-by: Zide Chen <zide.chen@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>
