xen-hvm: Clean up xen_hvm_init() error handling

xen-hvm: Clean up xen_hvm_init() error handling

xen_hvm_init() returns -1 without cleaning up on some errors (harmless
long as the caller exit()s on error), dies with hw_error() on others.
hw_error() isn't approprate here.  Clean up to exit() on all errors.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>