qemu: Fix get blkiodevtune for a disk that has been hot unplugged

qemu: Fix get blkiodevtune for a disk that has been hot unplugged

https://bugzilla.redhat.com/show_bug.cgi?id=1164080

After a disk is hotunplugged a subsequent call to qemuDomainGetBlockIoTune
to get the --config settings of that disk will fail because the disk is no
longer found by qemuDiskPathToAlias causing an unexpected failure.

Since only the --live flag needs to have the disk device pointer, move the
fetch inside the (flags & VIR_DOMAIN_AFFECT_LIVE) condition. This will also
affect the results if no flags are provided or the --current flag is provided.

Signed-off-by: Luyao Huang <lhuang@redhat.com>