vdsm: add logging to _getUnderlyingDriveInfo

_getUnderlyingDriveInfo() currently logs nothing - due
to unfrequent calls to this function, it might be good idea
to add verbose logging that displays how the devices are
matched in _devices and conf['devices']

Change-Id: I884e6b25c5bcb71ee24ad9ebad8df7c2775c5e19
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19928
Reviewed-by: Dan Kenigsberg <danken@redhat.com>