kexec: implement STATUS hypercall to check if image is loaded

kexec: implement STATUS hypercall to check if image is loaded

The tools that use kexec are asynchronous in nature and do not keep
state changes. As such provide an hypercall to find out whether an
image has been loaded for either type.

Note: No need to modify XSM as it has one size fits all check and
does not check for subcommands.

Note: No need to check KEXEC_FLAG_IN_PROGRESS (and error out of
kexec_status()) as this flag is set only once by the first/only
cpu on the crash path.

Note: This is just the Xen side of the hypercall, kexec-tools patch
to come separately.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>