stubdom: add videoram to stubdom initial memory reservation

stubdom: add videoram to stubdom initial memory reservation

This patch adds a videoram parameter (4MB is the default) to pv guests
too: if the pv guest is actually a stubdomain then the videoram size
is added to the initial required available memory.
This way there is no risk that a stubdom fails to populate the
videoram because the RAM freed for the videoram by dom0 has already
been used to create the stubdom.

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