Move the xsave area size calculation from cpu_x86_cpuid() inside
its own function. While doing it, change it to use the XSAVE area
struct sizes for the initial size, instead of the magic 0x240
number.
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>