tools/libxc: Prevent erroneous success from xc_domain_restore

tools/libxc: Prevent erroneous success from xc_domain_restore

The variable 'rc' is set to 1 at the top of xc_domain_restore, and for the
most part is left alone until success, at which point it is set to 0.

There is a separate 'frc' which for the most part is used to check function
calls, keeping errors separate from 'rc'.

For a toolstack which sets callbacks->toolstack_restore(), and the function
returns 0, any subsequent error will end up with code flow going to "out;",
resulting in the migration being declared a success.

For consistency, update the callsites of xc_dom_gnttab{,_hvm}_seed() to use
'frc', even though their use of 'rc' is currently safe.

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: George Dunlap <george.dunlap@eu.citrix.com>