Fix blkif.py for virsh start

Fix blkif.py for virsh start

I defined a cdrom to a guest domain as follows.

  ",hdc:cdrom,r"

When the guest domain was created by using xm create command, uname
of the cdrom was ''.  Then creating the guest domain succeeded.

[2008-06-03 15:23:41 xend.XendDomainInfo 25956] DEBUG
(XendDomainInfo:200)
XendDomainInfo.create(['vm', ['name', 'rhel5.2ga_hvm_lvol1'],
<<snip>>
['device', ['vbd', ['uname', ''], ['dev', 'hdc:cdrom'], ['mode',
'r']]],
<<snip>>

But, when the guest domain was created by using virsh start command,
uname of the cdrom was None.  Then creating the guest domain failed.

[2008-06-03 15:22:10 xend.XendDomainInfo 25559] DEBUG
(XendDomainInfo:200)
XendDomainInfo.create(['vm', ['name', 'rhel5.2ga_hvm_lvol1'],
<<snip>>
['device', ['vbd', ['dev', 'hdc:cdrom'], ['mode', 'r']]],
<<snip>>

This patch fixes to be able to create guest domains by both xm create
command and virsh start command.

FYI:
"if uname is None:" --> "if uname == '':" --> "if not uname:"
   till CS#17625             current           by this patch

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