Make qemuOpenFile aware of per-VM DAC seclabel.

Make qemuOpenFile aware of per-VM DAC seclabel.

Function qemuOpenFile() haven't had any idea about seclabels applied
to VMs only, so in case the seclabel differed from the "user:group"
from configuration, there might have been issues with opening files.

Make qemuOpenFile() VM-aware, but only optionally, passing NULL
argument means skipping VM seclabel info completely.

However, all current qemuOpenFile() calls look like they should use VM
seclabel info in case there is any, so convert these calls as well.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=869053