x86/hvm: Adjust hvmemul_rep_stos() to compile with -Wvla

x86/hvm: Adjust hvmemul_rep_stos() to compile with -Wvla

When using -Wvla, the typecast of buf triggers a Variable Length Array
warning.  This is less than ideal, as this typecast doesn't occupy any stack
space, but we don't have a finer grain option to use.

Alter the asm expression to avoid the typecast, which necessitates the
introduction of a memory clobber as the compiler can no longer identify
the total quantity of written memory.

Despite the memory clobber, there is no change to the generated asm.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>