blktap: Handle various tapdisk states

Until now the paused state was hardcoded as the '0x2a' value
which corresponds only to paused state that hasn't dropped the
logfile. This commit adds all possible tapdisk states and
uses those states to find if the block device is in paused state,
unpaused state or in pausing state.
blktap: Add new class static method

Add new static method for retrieving block device statistics.