monitor: Tidy up find_device_state()

Commit 6287d827 "monitor: allow device_del to accept QOM paths"
extended find_device_state() to accept QOM paths in addition to qdev
IDs.  This added a checked conversion to TYPE_DEVICE at the end, which
duplicates the check done for the qdev ID case earlier, except it sets
a *different* error: GenericError "ID is not a hotpluggable device"
when passed a QOM path, and DeviceNotFound "Device 'ID' not found"
when passed a qdev ID.  Fortunately, the latter won't happen as long
as we add only devices to /machine/peripheral/.

Earlier, commit b6cc36ab "qdev: device_del: Search for to be
unplugged device in 'peripheral' container" rewrote the lookup by qdev
ID to use QOM instead of qdev_find_recursive(), so it can handle
buss-less devices.  It does so by constructing an absolute QOM path.
Works, but object_resolve_path_component() is easier.  Switching to it
also gets rid of the unclean duplication described above.

While there, avoid converting to TYPE_DEVICE twice, first to check
whether it's possible, and then for real.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210916111707.84999-1-armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>