tools/libxc: Correct use of X86_XSS_MASK in guest xstate generation

tools/libxc: Correct use of X86_XSS_MASK in guest xstate generation

c/s 75f9455e "tools/libxc: Calculate xstate cpuid leaf from guest information"
incorrectly inverted the shift and mask when using X86_XSS_MASK.  Luckily, the
mask is currently zero, avoiding incorrect calculations.

While adjusting this, use an explcit uint32_t cast rather than masking against
0xffffffff.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>