tools/libxl: Split libxl__domain_create_state.restore_fd in two

tools/libxl: Split libxl__domain_create_state.restore_fd in two

In a future patch, we shall support automatically converting a legacy
stream to a v2 stream, in which case libxc needs to read from a
different fd.

Simply overwriting restore_fd does not work; the two fd's have
different circumstances.  The restore_fd needs to be returned to its
original state before libxl_domain_create_restore() returns, while in
the converted case, the fd needs allocating and deallocating
appropriately.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
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New in v2