feat(xenperf): add user interface

Add a function to list available events to the user.
Modify the main code so multiple counters can be enabled at the same
time.
Move sanity checks in a separated file.
fix(xenperf): detect pmc corruption in perfcnt_read

Before to read the MSR in perfcnt_read, the harware event selection
mask is read and compared to the expected one.
If their differ, a corruption is detected, the expected mask is
written again and an error code is returned.
Also remove the bitsize field in legacy_perfcnt for AMD/16/ since it
is a constant.
Merge branch 'xenperf-select-corruption' into xenperf
chore(xenperf): add average script for data transformation

Add a perl script to transform a per-core monitoring data into a
per-event monitoring data, making the average of each core for the
same event.
chore(xenperf): add gnuplot general data plotting

Add a perl script to generate gnuplot script and execute it according
to the input data and given options.
Merge branch 'xenperf-data-tool' into xenperf
fix(gnuplot): fix parameters handling on invocation

The gnuplot tool script can now specify options while taking its data
on stdin.
It was not possible before since it was trying to read the first
option as a data path.
It is now possible to bundle short options together.
Merge branch 'xenperf-gnuplot-invoc' into xenperf
docs(xenperf): add the number version

Add a number version, set in the Makefile and displayed after the
event list.
Merge branch 'release-1.0'