target-i386: make xmm_regs 512-bit wide

Right now, the AVX512 registers are split in many different fields:
xmm_regs for the low 128 bits of the first 16 registers, ymmh_regs
for the next 128 bits of the same first 16 registers, zmmh_regs
for the next 256 bits of the same first 16 registers, and finally
hi16_zmm_regs for the full 512 bits of the second 16 bit registers.

This makes it simple to move data in and out of the xsave region,
but would be a nightmare for a hypothetical TCG implementation and
leads to a proliferation of [XYZ]MM_[BWLSQD] macros.  Instead,
this patch marshals data manually from the xsave region to a single
32x512-bit array, simplifying the macro jungle and clarifying which
bits are in which vmstate subsection.

The migration format is unaffected.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>