savevm: Improve error message for blocked migration

savevm: Improve error message for blocked migration

If an internal snapshot can't be saved because migration is blocked
(most commonly probably because of AHCI), we had a really bad error
message:

$ echo -e "savevm foo\nquit" | qemu -M q35 /tmp/test.qcow2 -monitor stdio
QEMU 2.2.50 monitor - type 'help' for more information
(qemu) savevm foo
Error -22 while writing VM
(qemu) quit

This patch converts qemu_savevm_state() to the Error infrastructure so
that a useful error pointing to the problematic device is produced now:

$ echo -e "savevm foo\nquit" | qemu -M q35 /tmp/test.qcow2 -monitor stdio
QEMU 2.2.50 monitor - type 'help' for more information
(qemu) savevm foo
State blocked by non-migratable device '0000:00:1f.2/ich9_ahci'
(qemu) quit

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1423574702-23072-1-git-send-email-kwolf@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>