Add a new command dommemstats for virDomainMemoryStats

Add a new command dommemstats for virDomainMemoryStats

Define a new command 'dommemstats' to report domain memory statistics.  The
output format is inspired by 'domblkstat' and 'domifstat' and consists of
tag/value pairs, one per line.  The command can complete successfully and
print no output if virDomainMemoryStats is supported by the driver, but not
the guest operating system.

Sample output:
swap_in 0
swap_out 0
major_fault 54
minor_fault 58259
unused 487680
available 502472

All stats referring to a quantity of memory (eg. all above except major and
minor faults) represent the quantity in KBytes.

* tools/virsh.c: implements the new command