target-i386: use vmstate_offset_sub_array for AVX registers

After the next patch, each vmstate field will extract parts of a larger
(32x512-bit) array, so we cannot check the vmstate field against the
type of the array.

While changing this, change the macros to accept the index of the first
element (which will not be 0 for Hi16_ZMM_REGS) instead of the number
of elements (which is always CPU_NB_REGS).

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