ts-debian-hvm-install: Increase min guest ram size

ts-debian-hvm-install: Increase min guest ram size

768 MB for the guest isn't enough anymore for Debian Bookworm, at boot
it print this message:

    Low memory
    ----------

    Entering low memory mode

    This system has relatively little free memory, so it will enter low memory
    mode. Among other things, this means that this program will proceed in English.
    You should set up swap space as soon as possible.
    [Press enter to continue]

So, automatic installation fails. An empiric test shows that the min
was 817M for one particular situation (test-amd64-amd64-xl-qemuu-ovmf-amd64)
but we probably need some leg room, so increase to 1 GB.

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