Fix /domain/features setting in qemuParseCommandLine

Fix /domain/features setting in qemuParseCommandLine

Commit 5e6ce1 moved down detection of the ACPI feature in
qemuParseCommandLine. However, when ACPI is detected, it clears
all feature flags in def->features to only set ACPI. This used to
be fine because this was the first place were def->features was set,
but after the move this is no longer necessarily true because this
block comes before the ACPI check:

if (strstr(def->emulator, "kvm")) {
    def->virtType = VIR_DOMAIN_VIRT_KVM;
    def->features |= (1 << VIR_DOMAIN_FEATURE_PAE);
}

Since def is allocated in qemuParseCommandLine using VIR_ALLOC, we
can always use |= when modifying def->features