hw/i386/pc: use PVH option rom

Use pvh.bin option rom when we are booting an uncompressed
kernel using the x86/HVM direct boot ABI.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Based-on: <1547554687-12687-1-git-send-email-liam.merwick@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>