xen: use errno instead of rc for xc_domain_add_to_physmap

In Xen 4.6 commit cd2f100f0f61b3f333d52d1737dd73f02daee592
"libxc: Fix do_memory_op to return negative value on errors"
made the libxc API less odd-ball: On errors, return value is
-1 and error code is in errno. On success the return value
is either 0 or an positive value.

Since we could be running with an old toolstack in which the
Exx value is in rc or the newer, we add an wrapper around
the xc_domain_add_to_physmap (called xen_xc_domain_add_to_physmap)
which will always return the EXX.

Xen 4.6 did not change the libxc functions mentioned (same parameters)
so we piggyback on the fact that Xen 4.6 has a new function:
commit 504ed2053362381ac01b98db9313454488b7db40 "tools/libxc: Expose
new hypercall xc_reserved_device_memory_map" and check for that.

Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Suggested-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
