hmp: delvm: use hmp_handle_error

This gives us the consistent 'Error:' prefix added in 66363e9a,
which helps users like libvirt who still need to scrape hmp error
messages to detect failure.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Message-Id: <984634226fa14316641f31f84e7dd0bb770bd94c.1556141033.git.crobinso@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>