Fix virDomainDeviceInfoIsSet() to check all struct fields

Fix virDomainDeviceInfoIsSet() to check all struct fields

The virDomainDeviceInfoIsSet API was only checking if an
address or alias was set in the struct. Thus if only a
rom bar setting / filename, boot index, or USB master
value was set, they could be accidentally dropped when
formatting XML
(cherry picked from commit 2c195fdbf3293792e39bc4f06755ae751025b0ea)

Conflicts:

	src/conf/domain_conf.c

(crobinso: some elements aren't in maint branch, drop them)