pyGrub: Use proper bootloader class when entering command manually

pyGrub: Use proper bootloader class when entering command manually

Use the proper bootloader class when entering the boot commands
manually (i.e. using the 'c' option). Before this patch the bootloader
was always treated to be Grub but when user is using Grub2/ExtLinux or
Lilo it's rather confusing. After applying this patch the proper
bootloader image class is being used, e.g. Grub2Image for Grub2
etc. when you define the boot commands manually using the 'c' command
in pyGrub.

Also, fix for using isconfig has been applied since if there is not fs
set in the run_grub() method the read_config() would fail since it's
trying to access undefined self.cf which is now being set to parser()
from cfg_list.

Signed-off-by: Michal Novotny <minovotn@redhat.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>