Allow the Xen drivers to determine default vram values.  Sane
default vaules depend on the device model being used, so the
drivers are in the best position to determine the defaults.

For the legacy xen driver, it is best to maintain the existing
logic for setting default vram values to ensure there are no
regressions.  The libxl driver currently does not support
configuring a video device.  Support will be added in a
subsequent patch, where the benefit of this change will be
reaped.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>