qemu: agent: use GetArray to remove a check

qemu: agent: use GetArray to remove a check

The error check for ValueObjectGet("return") is redundant,
qemuAgentCommand already checked for us that the reply contains
a "return" object.

It does not guarantee, that it is an array.

Use virJSONValueObjectGetArray that combines getting the object
with checking for its type and return the more helpful of
the two error messages.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>