vmspawn-qmp: add vmspawn_qmp_remove_block_device

vmspawn-qmp: add vmspawn_qmp_remove_block_device

Hot-remove counterpart to vmspawn_qmp_add_block_device. Looks the
drive up in the bridge's block_devices registry by caller-supplied
id and dispatches device_del using the internal qmp_device_id; the
varlink link gets the immediate ack/error reply once QEMU completes
the request.

Concurrency: a second remove for the same id while the first is in
flight (between device_del dispatch and DEVICE_DELETED) would
otherwise reach QEMU and earn a confusing 'already in the process of
unplug' reply. Track the in-flight state with a new
BLOCK_DEVICE_REMOVE_PENDING bit on the existing rollback_mask, and
short-circuit duplicate calls with -EBUSY. The bit is cleared on
device_del failure (the drive is still attached, so retries make
sense) and naturally vanishes on success when the registry entry is
dropped.

DEVICE_DELETED handling: the actual blockdev-del + registry removal
+ pcie-port release is deferred to vmspawn_qmp_dispatch_device_deleted,
which fires from on_qmp_event in vmspawn-varlink.c when the guest
acks the eject. Hooking it from the existing QMP event dispatcher
keeps the cleanup local to vmspawn-qmp.{c,h}.

The function has no varlink callers in this PR — the
io.systemd.VirtualMachineInstance method handler that forwards into
it lands with the rest of the hotplug PR.

Signed-off-by: Christian Brauner <brauner@kernel.org>