virt-install: add support for OVMF

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
virt-install: add tests for OVMF

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
virt-manager: OVMF support

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
virt-manager: delete nvram file on VM deletion

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>