(1) send SPICE_MSG_MAIN_MIGRATE_BEGIN upon spice_server_migrate_connect
(2) wait for SPICE_MSGC_MAIN_MIGRATE_(CONNECTED|CONNECT_ERROR), or a timeout, in order
    to complete client_migrate_info monitor command

They were trying to convert the destination pointer to an integer before
trying to dereference it. The initial conversion was meant to be a cast
to a pointer of the right size, not to an integer.

uint63_t should be uint64_t

In C, the latter isn't a prototype for a function with no arg,
but declares a function with an undefined number of args.

[picked from master with changes since no main_channel, spice_common,
 and a bunch of functions aren't there yet]

(this patch is not to solve a crash fix, but to align with glib API)

Since some spice C++ code is using code from common/, the C
functions need to be marked as such for the C++ compiler, otherwise
we'll get linkage issues.

They were detected using clang-static-analyzer. Don't initialize
the variable to a value to override it with a different value
a few lines after.

clang static analyzer warned that 'len' was computed but never
used in glc_vertex2d. glc_stroke_line_dash has side effects so
we have to call it, but we don't need to save its return value
since it's not used.

https://bugs.freedesktop.org/show_bug.cgi?id=34795

This is the only unused var change I'll want to revisit eventually,
I'm submitting anyway since it doesn't change current behavior. I'm
talking about ignoring the return value from canvas creation. Adding
a print is possible but I didn't test (may be too verbose, also
preferable to be a debug print if so, and we don't have that option
in the code atm - probably an environment variable will do, or adding
some spice_server_set_logging_level api, maybe even
spice_server_set_logging_fd?)