plat/kvm: Expand `KVM_BOOT_PROTO_LXBOOT` configuration entry

plat/kvm: Expand `KVM_BOOT_PROTO_LXBOOT` configuration entry

Add a description and proper dependencies for the
`KVM_BOOT_PROTO_LXBOOT` configuration entry. Since `Firecracker`
only supports the `Linux` 64-bit boot protocol and we do not yet
support booting through it on `QEMU`, make the dependencies
reflect that.

Signed-off-by: Sergiu Moga <sergiu.moga@protonmail.com>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #909