x86/hvm: Fixes to hvmemul_insn_fetch()

x86/hvm: Fixes to hvmemul_insn_fetch()

Force insn_off to a single byte, as offset can wrap around or truncate with
respect to sh_ctxt->insn_buf_eip under a number of normal circumstances.

Furthermore, don't use an ASSERT() for bounds checking the write into
hvmemul_ctxt->insn_buf[].

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>