Unvalidated completion context and ring log2 sizes from hypervisor-controlled shared DMA memory

Summary

pvscsi.c:899 context_to_hcb(e->context) no check vs hcb_cnt. context=0 -> uint64 underflow. :1049/:1126 cmp/msg_num_entries_log2 from shared DMA used as ring mask, no validation vs allocated ring. :1306 req_num_entries_log2 same. Hypervisor controls these fields. OOB on ring buffers or wild hcb pointer. Defense-in-depth (hypervisor already trusted). Fix: validate context in [1,hcb_cnt], check log2 vs ring allocation.