fix return check for KVM_GET_DIRTY_LOG ioctl

Fix return condition check from kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d) to
handle internal failures or no support for memory slot dirty bitmap.
Otherwise the ioctl succeeds and continues with migration.
Addresses BUG# 1294227

Signed-off-by: Mario Smarduch <m.smarduch@samsung.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
