check for stage1 when not installing bootloader (#882065,#895232)
check for stage1 when not installing bootloader (#882065,#895232)

When not installing a bootloader stage1_device may not be set, need to check
before using it.

rhinstaller/blivet,jkonecny12/blivet,rhinstaller/blivet,dwlehman/blivet,AdamWill/blivet,rvykydal/blivet,vpodzime/blivet,vojtechtrefny/blivet,rvykydal/blivet,vpodzime/blivet,jkonecny12/blivet,dwlehman/blivet,AdamWill/blivet,vojtechtrefny/blivet