hvm: Fix undefined bit shifting in mmio emulation path

hvm: Fix undefined bit shifting in mmio emulation path

In functions set_eflags_* (xen/arch/x86/hvm/io.c), if the first
argument "size" equals sizeof(long), the following code will produce
unintended and invalid result:
        unsigned long mask = (1 << (8 * size)) - 1;

In ANSI C, if the shift amount is greater or equal to the width of the
data type, the result is undefined. Specifically on x86, a bit mask is
applied to the shift amount, so that more significant bits are
ignored. So the above expression results 0x0 instead of the intended
~0UL.

This patch fixes this issue. Because size=0 is not a valid parameter,
rewriting the code using right shift avoids an additional condition
check.

Signed-off-by: Qing He <qing.he@intel.com>