Set default emulator in domain_conf.*, rather than the driver.

Set default emulator in domain_conf.*, rather than the driver.

Rather than numerous instances of:

emulator = vm->def->emulator;
if (!emulator)
    emulator = virDomainDefDefaultEmulator(conn, vm->def, driver->caps);
if (!emulator)
    return -1;

Set this value at XML parse time in the domain config, so we can depend on
it for all future emulator accesses. There were unchecked accesses in the
qemu driver that were tripping up on this if no emulator was specified in
the XML, see:

http://www.redhat.com/archives/libvir-list/2008-October/msg00602.html