qemu: return -1 on error paths in qemuDomainSaveImageStartVM

qemu: return -1 on error paths in qemuDomainSaveImageStartVM

Error paths after sending the event that domain is started written as if ret = -1
which is set at the beginning of the function. It's common idioma to keep 'ret'
equal to -1 until the end of function where it is set to 0. But here we use ret
to keep result of restore operation too and thus breaks the idioma and its users :)

Let's use different variable to hold restore result.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>