qemu: Fix error reporting in qemuDomainSaveImageStartVM

qemu: Fix error reporting in qemuDomainSaveImageStartVM

When restoring a domain from a compressed image, we launch an
intermediate process for decompressing the saved data. If QEMU fails to
load the data for some reason, we force close the stdin/stdout file
descriptors of the intermediate process and wait for it to die. However,
virCommandWait can report various errors which would overwrite the real
error from QEMU. Thus instead of getting something useful:

    internal error: process exited while connecting to monitor:
    2018-09-17T15:17:29.998910Z qemu-system-x86_64: can't apply global
    Skylake-Client-x86_64-cpu.osxsave=off: Property '.osxsave' not found

we could get an irrelevant error message:

    internal error: Child process (lzop -dc --ignore-warn) unexpected
    fatal signal 13

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>