ArmVirtPkg: do not enable iSCSI driver by default

ArmVirtPkg: do not enable iSCSI driver by default

The iSCSI driver slows down the boot on a pristine variable store flash
image, as it creates a couple of large EFI non-volatile variables to
preserve state between boots.

Since iSCSI boot for VMs is kind of niche anyway, let's default to
disabled. If someone needs it in their build, they can use the -D build
command option to re-enable it on the fly.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>