tools/libs/light: set video_mem for PVH guests

tools/libs/light: set video_mem for PVH guests

The size of the video memory of PVH guests should be set to 0 in case
no value has been specified.

Doing not so will leave it to be -1, resulting in an additional 1 kB
of RAM being advertised in the memory map (here the output of a PVH
Mini-OS boot with 16 MB of RAM assigned):

Memory map:
000000000000-0000010003ff: RAM
0000feff8000-0000feffffff: Reserved
0000fc008000-0000fc00803f: ACPI
0000fc000000-0000fc000fff: ACPI
0000fc001000-0000fc007fff: ACPI

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>