run: Check for ghost socket file upon VM creation

Kvmtool creates a (debug) UNIX socket file for each VM, using its
(possibly auto-generated) name as the filename. There is a check using
access(), which bails out with an error message if a socket with that
name already exists.

Aside from this check being unnecessary, as the bind() call later would
complain as well, this is also racy. But more annoyingly the bail out is
not needed most of the time: an existing socket inode is most likely just
an orphaned leftover from a previous kvmtool run, which just failed to
remove that file, because of a crash, for instance.

Upon finding such a collision, let's first try to connect to that socket,
to detect if there is still a kvmtool instance listening on the other
end. If that fails, this socket will never come back to life, so we can
safely clean it up and reuse the name for the new guest.
However if the connect() succeeds, there is an actual live kvmtool
instance using this name, so not proceeding is the only option.
This should never happen with the (PID based) automatically generated
names, though.

This avoids an annoying (and not helpful) error message and helps
automated kvmtool runs to proceed in more cases.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>