coverage: Add command for reading counter value.

coverage: Add command for reading counter value.

From: Jakub Sitnicki <jkbs@redhat.com>

Facilitate checking coverage counters from scripts and tests with a new
coverage/read-counter command that gets the total count for a counter.

Same could be achieved by scraping the output of coverage/show command
but the difficulties there are that output is in human readable format
and zero-value counters are not listed.

Signed-off-by: Jakub Sitnicki <jkbs@redhat.com>
Acked-by: Han Zhou <hzhou8@ebay.com>
Signed-off-by: Han Zhou <hzhou8@ebay.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>