Because stage2 does not include the udev 60-blocks.rule, udev does not
automatically update the soft links in /dev/disk/by-uuid after we write
the partition table and format the partitions. We need these links to
be updated before we create the initrd. It would be cleaner to fix this
with a udev rule, but for now, use brute force.

Thus ensuring we get the correct device numbers after writing a
partition table (mga#22032).

This enables bootloader::read_grub2 to recognise the initrd16 keyword,
which is used by default on x86 machines.

The video mode needs to be extracted from and written back to the
GRUB_CMDLINE_LINUX_DEFAULT parameter in /etc/defaults/grub2.

This fix creates a new "default_vga" key in the $bootloader hash to
hold the current selection. Using the existing "vga" key would cause
the default video mode to be added to any bootloader entries that
don't have a specific "vga" value, e.g. the "failsafe" entry.

The $bootloader "vga" value doesn't appear to be set anywhere else,
so may in fact be obsolete.

This reverts commit 7bb01fb038e0a882e755b963110bda381336b748 and
commit 4c1cb5006360ccb780926000bd57f150e0fc8fbe