However if I set the same CPU capabilities as libvirt does (due to <cpu mode='host-model'>)
the kernel Oops'es while trying to use avx2 instructions:

CPUFLAGS="-cpu Nehalem,+invpcid,+erms,+bmi2,+smep,+avx2,+bmi1,+fsgsbase,+abm,+rdtscp,+pdpe1gb,+rdrand,+f16c,+avx,+osxsave,+xsave,+tsc-deadline,+movbe,+x2apic,+dca,+pcid,+pdcm,+xtpr,+fma,+tm2,+est,+smx,+vmx,+ds_cpl,+monitor,+dtes64,+pclmuldq,+pbe,+tm,+ht,+ss,+acpi,+ds,+vme -smp 1,sockets=1,cores=1,threads=1"

qemu-system-x86_64 -enable-kvm \
        -m 4096 $CPUFLAGS \
        -hda xenial-tmp.qcow2 \
        -kernel xenial-server-cloudimg-amd64-vmlinuz-generic \
        -initrd xenial-server-cloudimg-amd64-initrd-generic \
        -append 'root=/dev/sda1 console=ttyS0 init=/bin/bash' \
        -machine pc-1.0,accel=kvm,usb=off \
        -rtc base=utc \
        -serial mon:stdio \
        -vga none -nographic


Begin: Loading essential drivers ... [   33.325464] md: linear personality registered for level -1
[   33.329338] md: multipath personality registered for level -4
[   33.332992] md: raid0 personality registered for level 0
[   33.337063] md: raid1 personality registered for level 1
[   33.408007] raid6: sse2x1   gen()  8742 MB/s
[   33.476005] raid6: sse2x1   xor()  7756 MB/s
[   33.544008] raid6: sse2x2   gen() 12452 MB/s
[   33.612004] raid6: sse2x2   xor()  8569 MB/s
[   33.680005] raid6: sse2x4   gen() 14347 MB/s
[   33.748005] raid6: sse2x4   xor()  9526 MB/s
[   33.752013] invalid opcode: 0000 [#1] SMP 
[   33.752376] Modules linked in: raid6_pq(+) libcrc32c raid1 raid0 multipath linear psmouse floppy e1000
[   33.753319] CPU: 0 PID: 610 Comm: modprobe Not tainted 4.4.0-34-generic #53-Ubuntu
[   33.753978] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   33.754498] task: ffff8800d92acb00 ti: ffff8800dacac000 task.ti: ffff8800dacac000
[   33.755151] RIP: 0010:[<ffffffffc0086a8d>]  [<ffffffffc0086a8d>] raid6_avx21_gen_syndrome+0x3d/0x120 [raid6_pq]
[   33.756002] RSP: 0018:ffff8800dacafb78  EFLAGS: 00010246
[   33.756002] RAX: 0000000000000000 RBX: ffff8800dacafbc8 RCX: 00000000fffefbfe
[   33.756002] RDX: 0000000000000080 RSI: 0000000000001000 RDI: 0000000000000012
[   33.756002] RBP: ffff8800dacafba8 R08: 000000000000000a R09: 00000000000001b6
[   33.756002] R10: 00000000fffefbed R11: 00000000000001b6 R12: 0000000000001000
[   33.756002] R13: ffff8800d928e000 R14: ffff8800d928f000 R15: 0000000000000012
[   33.756002] FS:  00007f5cdbac3700(0000) GS:ffff88011fc00000(0000) knlGS:0000000000000000
[   33.756002] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   33.756002] CR2: 000055e32b7d3180 CR3: 00000000d9314000 CR4: 00000000001006f0
[   33.756002] Stack:
[   33.756002]  0000000000000080 ffffffffc00997a0 0000000000000001 0000000000004c36
[   33.756002]  ffffffffc0087238 ffff8800d928e000 ffff8800dacafc88 ffffffffc009e116
[   33.756002]  00000000fffefbfe 0000000000003964 ffffffffc0089600 ffffffffc008a600
[   33.756002] Call Trace:
[   33.756002]  [<ffffffffc009e116>] init_module+0x116/0x1000 [raid6_pq]
[   33.756002]  [<ffffffffc009e000>] ? 0xffffffffc009e000
[   33.756002]  [<ffffffff81002123>] do_one_initcall+0xb3/0x200
[   33.756002]  [<ffffffff8182a018>] ? preempt_schedule_common+0x18/0x30
[   33.756002]  [<ffffffff8182a04c>] ? _cond_resched+0x1c/0x30
[   33.756002]  [<ffffffff811eb843>] ? kmem_cache_alloc_trace+0x183/0x1f0
[   33.756002]  [<ffffffff8118c763>] do_init_module+0x5f/0x1cf
[   33.756002]  [<ffffffff8110a1c7>] load_module+0x1667/0x1c00
[   33.756002]  [<ffffffff81106770>] ? __symbol_put+0x60/0x60
[   33.756002]  [<ffffffff81213180>] ? kernel_read+0x50/0x80
[   33.756002]  [<ffffffff8110a9a4>] SYSC_finit_module+0xb4/0xe0
[   33.756002]  [<ffffffff8110a9ee>] SyS_finit_module+0xe/0x10
[   33.756002]  [<ffffffff8182def2>] entry_SYSCALL_64_fastpath+0x16/0x71
[   33.756002] Code: 55 41 54 53 48 89 d3 48 8d 14 c5 00 00 00 00 41 89 ff 49 89 f4 48 83 ec 08 4c 8b 2c c3 4c 8b 74 13 08 48 89 55 d0 e8 f3 32 fb c0 <c5> fd 6f 05 8b 2e 01 00 c5 e5 ef db 4d 85 e4 48 8b 55 d0 0f 84 
[   33.756002] RIP  [<ffffffffc0086a8d>] raid6_avx21_gen_syndrome+0x3d/0x120 [raid6_pq]
[   33.756002]  RSP <ffff8800dacafb78>
[   33.773948] ---[ end trace 031120da9402feef ]---