Make check for /dev/loop device names stricter to avoid /dev/loop-control

Make check for /dev/loop device names stricter to avoid /dev/loop-control

Recentish (2011) kernels introduced a new device called /dev/loop-control,
which causes libvirt's detection of loop devices to get confused
since it only checks for a prefix of 'loop'. Also check that the
next character is a digit

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>