loader: Handle memory-mapped ELFs

This patch allows handling an ELF memory-mapped, taking care
the reference count of the GMappedFile* passed through
rom_add_elf_program().
In this case, the 'data' pointer is not heap-allocated, so
we cannot free it.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20190724143105.307042-2-sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>