QHostInfo: replace a volatile bool with an atomic int

QHostInfo: replace a volatile bool with an atomic int

A volatile bool read/store is documented on MSVC to have acquire/release
semantics, respectively, but that doesn't need to be true for MinGW, so
use explicit memory ordering.

Apply the same fix to the Unix implementation, too.

Change-Id: Ica466cec50beed830aafa4e3384d82f02e1a47e8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>