We reused host EBX and ECX, but KVM might not support all features;
emulated XSAVE size should be smaller.

EBX depends on unknown XCR0, so we default to ECX.

SDM CPUID (EAX = 0DH, ECX = 0):
 EBX Bits 31-00: Maximum size (bytes, from the beginning of the
     XSAVE/XRSTOR save area) required by enabled features in XCR0. May
     be different than ECX if some features at the end of the XSAVE save
     area are not enabled.

 ECX Bit 31-00: Maximum size (bytes, from the beginning of the
     XSAVE/XRSTOR save area) of the XSAVE/XRSTOR save area required by
     all supported features in the processor, i.e all the valid bit
     fields in XCR0.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Tested-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>