xend: Do not overwrite xauthority and display with empty values

xend: Do not overwrite xauthority and display with empty values

Display and xauthority vars are read from vmConfig['platform'] first,
then they are read again from dev_info.
However if the user does not set those variable in the config file,
dev_info won't contain them, hence we are going to overwrite the
current significant values with null.
This patch fixes the problem setting display and xauthority to the
current values if dev_info does not contain them.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>