Most code paths for creating a user creatable object go through
QemuOpts, which ensures that the provided 'id' option is actually a
valid identifier.

However, there are some code paths that don't go through QemuOpts:
qemu-storage-daemon --object (since commit 8db1efd3) and QMP object-add
(since it was first introduced in commit cff8b2c6). We need to have the
same validity check for those, too.

This adds the check and makes it print the same error message as
QemuOpts on failure.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210302171623.49709-1-kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>