CA-149017: libxl will reject 4MiB videoram with upstream qemu

If the user asks for 4MiB (-- which is the xapi default, see
[xapi-project/xen-api#1989]) then, if xenopsd is using upstream
qemu, then it will emit a warning and transparently increase the
amount of videoram to the minimum (i.e. 8MiB)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>