qemu: Implement extended loader and nvram

qemu: Implement extended loader and nvram

QEMU now supports UEFI with the following command line:

  -drive file=/usr/share/OVMF/OVMF_CODE.fd,if=pflash,format=raw,unit=0,readonly=on \
  -drive file=/usr/share/OVMF/OVMF_VARS.fd,if=pflash,format=raw,unit=1 \

where the first line reflects <loader> and the second one <nvram>.
Moreover, these two lines obsolete the -bios argument.

Note that UEFI is unusable without ACPI. This is handled properly now.
Among with this extension, the variable file is expected to be
writable and hence we need security drivers to label it.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>