server: always call read_from_vdi_port() in a while loop

read_from_vdi_port() MUST always be called in a while loop until it returns 0.

This is needed because it can cause new data available events and its
recursion protection causes those to get lost. Calling it until it returns 0
ensures that all data has been consumed.

Example scenario where we can fail to read the available data otherwise:
- server/reds.c: vdagent_char_device_wakeup get called
  by hw/spice-vmc.c because data has arrived from the guest,
- hw/spice-vmc.c: vmc_read get calls
- If the vmc_read call depletes the current buffer it calls
  virtio_serial_throttle_port(&svc->port, false)
- This causes vmc_have_data to get called, which if in the
  mean time another buffer has arrived causes
  vdagent_char_device_wakeup to gets called again
  (so recursively)
- vdagent_char_device_wakeup is protected against recursive
  calling and ignores the second call (a nasty hack)
- if no other data arrives, the arrived data will not get read

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