hw/i386/pc: run the multiboot loader before the PVH loader

hw/i386/pc: run the multiboot loader before the PVH loader

Some multiboot images could be in the ELF format. In the current
implementation QEMU fails because we try to load these images
as a PVH image.

In order to fix this issue, we should try multiboot first (we
already check the multiboot magic header before to load it).
If it is not a multiboot image, we can try the PVH loader.

Fixes: ab969087da6 ("pvh: Boot uncompressed kernel using direct boot ABI", 2019-01-15)
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20190214180216.246707-1-sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>