fastboot: Fix IPv6 connect and -s host parsing

During fastboot connect / disconnect introduction, we
completely broke the IPv6 support (it was considering
all IPv6 addresses as a USB serial).

Makeing sure this problem isn't reproducible anymore
alongside with fixing EXCPECT causing process crash
and improve network serial error detection.

Bug: 271152365
Change-Id: Ic52aa5fff1948a64ac3d2672f3cf4d2b022e5cea
Signed-off-by: Dmitrii Merkurev <dimorinny@google.com>