x86/xen: remove unneeded dummy push from xen_hypercall_hvm()

Stack alignment of the kernel in 64-bit mode is 8, not 16, so the
dummy push in xen_hypercall_hvm() for aligning the stack to 16 bytes
can be removed.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>