xend: Add syntax check for block devices

If we define wrong values to a disk parameter in domain configuration
files, we get an error message or a guest OS panic.

1. If we define a wrong disk type, xm create command error occurs
about 100 seconds later.
e.g. disk=['xyz:/xen/root-vm1.img,hda1,w']

2. If we forget a disk type, a guest OS panic occurs.
e.g. disk=['/xen/root-vm1.img,hda1,w']

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>