qemu: Start nested job in qemuDomainCheckEjectableMedia
    
    Originally, qemuDomainCheckEjectableMedia was entering monitor with qemu
    driver lock. Commit 2067e31bf97667eab9f111b496f5e9a44e827c5b, which I
    made to fix that, revealed another issue we had (but didn't notice it
    since the driver was locked): we didn't set nested job when
    qemuDomainCheckEjectableMedia is called during migration. Thus the
    original fix I made was wrong.