bluestore/NVMEDevice: do not deference a dangled pointer

bluestore/NVMEDevice: do not deference a dangled pointer

* pass coremask_arg to the working thread by value.
  after 81249ab9d, get_val<>() returns a temporary variables instead of a
  reference to the variant held by config. so calling string::c_str(), and
  passing the returned `const char*` to a thread is not advisable. instead
  we should pass a string by value. since spdk_env_init() will copy the
  settings passed in by opt. it's safe even to destruct the coremask_arg
  afterward.
* use ffsll() to find the LSB. it's not a bottleneck, but it's easier
  and simpler, and probably improves the readability.
* refactor the NVMEManager::try_get() method: to define the variables
  when they are used for the first time.

Signed-off-by: Kefu Chai <kchai@redhat.com>