vgdb: Handle EAGAIN in read_buf

vgdb: Handle EAGAIN in read_buf

The file descriptor is on non-blocking mode and read_buf should only
be called when poll gave us an POLLIN event signaling the file
descriptor is ready for reading from. Still sometimes we do get an
occasional EAGAIN. Just do as told in that case and try to read again.

Also fix an ERROR errno in getpkt. This has never been observed, but
not getting the actual errno if the write fails in that case would be
really confusing.