hvm: Allow blktap to be able to be booted as system volume for PV-on-HVM

hvm: Allow blktap to be able to be booted as system volume for PV-on-HVM

When blktap was booting system volume, PV-on-HVM domain was not able
to be  started.

The configuration file is specified as follows.
disk = [ 'tap:aio:/xen/test/rhel5ga_full.img,hda,w' ]

The error occurred by the initialization of system volume in qemu-dm.
qemu: could not open hard disk image 'aio:/xen/test/rhel5ga_full.img'

Signed-off-by: Takanori Kasai <kasai.takanori@jp.fujitsu.com>
Signed-off-by: Tomonari Horikoshi <t.horikoshi@jp.fujitsu.com>
Signed-off-by: Tsunehisa Doi <Doi.Tsunehisa@jp.fujitsu.com>
Signed-off-by: Keir Fraser <keir@xensource.com>