qemu 4.0.0 will prefix most errors with 'Error: ', so consider any
string instance of that an error.

This fixes savevm failure detection when migration is blocked due to
usage of nested VMX

https://bugzilla.redhat.com/show_bug.cgi?id=1697997Acked-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>