KVM: Abstract/rework instance up checks
KVM: Abstract/rework instance up checks

This patch abstract the check "is instance stopped" into a separate
function, and thus simplifies a couple of higher-level functions. It
also moves from manual read of the pidfile to use the (correct
abstraction of) _InstancePidAlive.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Olivier Tharan <0947fcc917eb1d3c89ad818beb61e3b2c3cf3bba@google.com>

ganeti/ganeti,grnet/snf-ganeti,ganeti/ganeti,mbakke/ganeti,onponomarev/ganeti,andir/ganeti,bitemyapp/ganeti,apyrgio/ganeti,dimara/ganeti,yiannist/ganeti,ganeti/ganeti,dimara/ganeti,mbakke/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,grnet/snf-ganeti,apyrgio/ganeti,andir/ganeti,yiannist/ganeti,leshchevds/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,apyrgio/snf-ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,yiannist/ganeti,andir/ganeti