libvirt: virsh: Kill all uses of __FUNCTION__ in error messages

libvirt: virsh: Kill all uses of __FUNCTION__ in error messages

The error output of snapshot-revert should be more friendly.
There is no need to show virDomainRevertToSnapshot to user.
virReportError already includes __FUNCTION__ information in a
separate member of the struct, so repeating it in the message is
redundant and leads to situations where higher level code ends up
reporting the lower level name. We correctly converted the error
output making it more succinct and user-friendly.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1086726