crypto-ipsecmb: fix plugin crash in VirtualBox

crypto-ipsecmb: fix plugin crash in VirtualBox

Plugin checks just for AVX2 instruction set, while the v1.3 of IPsec
Multi-Buffer library checks for both AVX2 and BMI2 sets during init.
VirtualBox VM doesn't provide BMI2 by default to guest operating system.

Result is that VPP plugin decides to use AVX2 initialization and library
then doesn't do it. Since flush_job remains empty, the self-check fails
and with that the whole VPP crashes on start-up.

Type: fix
Signed-off-by: Maros Ondrejicka <maros.ondrejicka@pantheon.tech>
Change-Id: I6b661f2b9bbe6dd03b499c55c38a9b814e6d718a