build: switch from SYSLINUX to GRUB2

The new grub install script must be called after the image is unmounted
and the old bootloaders script doesn't need to touch grub at all. For
now we will continue to use the existing syslinux configs but
interpreted by grub. Beyond the grub menu flashing by during boot
everything should still be functionally equivalent.
qemu: adjust the meaning of the -s (simple) flag

My primary use case for this flag is to fix booting with UEFI firmware
which can have problems when mixed with KVM, adding kexec into the mix
doesn't help matters either. The current version of OVMF can boot from
virtio drives just fine so that is now enabled and KVM is disabled.

So the -s option can also mean sloooooooow but boots!