xen/hvc: Fix error cases around HVM_PARAM_CONSOLE_PFN

commit a32c88b9386ce3df87f28dd46bdc3776cd6edf75 upstream.

We weren't resetting the parameter to be passed in to a
known default. Nor were we checking the return value of
hvm_get_parameter.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>