xen-block: report error condition from vbd_name_to_disk()

xen-block: report error condition from vbd_name_to_disk()

The function needs to make sure it is passed a valid disk name. This is
easily done by making sure that the parsing loop results in a non-zero
value.

Spotted by Coverity: CID 1398640

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20190215162533.19475-4-paul.durrant@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>