Load global config files by default

A new option, -nodefconfig is introduced to prevent loading from the default
config location.  Otherwise, two configuration files will be searched for,
qemu.conf and target-<TARGET_NAME>.conf.

To ensure that the default configuration is overridden by a user specified
config, we introduce a two stage option parsing mechanism.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
