Fix generation of marshallers in VPATH builds

The rules to generate the .c/.h (de)marshalling files have targets based in
$builddir, but the CLIENT_MARSHALLERS/SERVER_MARSHALLERS list refer to
files in $srcdir.
When using a $srcdir != $builddir, these $srcdir files will not exist, and
it will not be possible to generate them, which causes the build to fail.

When using $srcdir == $builddir from git or from tarballs, this will not
make a difference. When building from git, if $srcdir and $builddir are the
same, then the files will be found regardless of if we look for them in
$srcdir or $builddir as they are the same.
In tarballs, the files will be shipped with the tarball and thus available
in $srcdir. As $builddir is the same as $srcdir, the files will already
exist and not be regenerated.

The only change of behaviour will be when using a tarball and doing a
$srcdir != $builddir build. In this case, the files will need to be
regenerated, so the tools needed for that must be installed on the
machine doing the build.
Use #include "common/..." in (de)marshallers

Now that they are created in $builddir, their includes will need to refer
to files in $srcdir, which can be different. It's cleaner to add
-I $(top_srcdir)/spice-common/ to modules using spice-common rather than
having -I $(top_srcdir)/spice-common/common which would could create header
collisions.
marshaller: Use #include <> for headers in $srcdir/common

Since the (de)marshallers are now generated in $builddir and not in
$srcdir, when these generated files include a file located in
$srcdir/common, the compiler will find them thanks to a -I directive, so it
makes more sense to use <> rather than "" when including them.
Mark sw_canvas.[ch] as automake source files

These 2 files are not build as part of spice-common
build system, but modules using spice-common will build
them with the appropriate options. We need to let automake
know that these are source files so that it can properly
track these files dependencies.

Without this change, when eg spice-gtk switches to use 'subdir-objects'
to build sw_canvas.c as recommended by automake 1.14, the build
will fail because $(top_srcdir)/spice-common/common/.deps/sw_canvas.Plo
will not have been generated.
demarshal: prefix variable name with parent names

Avoid naming clash if parent structures have the same name

https://bugzilla.redhat.com/show_bug.cgi?id=1021995