elf-loader: Allow late loading of elf

The current QEMU ROM infrastructure rejects late loading of ROMs.
And ELFs are currently loaded as ROM, this prevents delayed loading
of ELFs. So when loading ELF, allow the user to specify if ELF should
be loaded as ROM or not.

If an ELF is not loaded as ROM, then they are not restored on a
guest reboot/reset and so its upto the user to handle the reloading.

Signed-off-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
