xenbus: avoid uninitialized variable warning

Older compilers don't recognize that "v" can't be used uninitialized;
other code using hvm_get_parameter() zeros the value too, so follow
suit here.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>